Patent Publication Number: US-9841711-B2

Title: Image forming apparatus, storage medium and method for controlling image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2015-152336 filed on Jul. 31, 2015, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The disclosure relates to an image forming apparatus including a fixing device including a heater, a storage medium storing a program that is to be executed by a computer configured to control the image forming apparatus, and a method for controlling an image forming apparatus. 
     BACKGROUND 
     In the related art, an image forming apparatus including a fixing device has been known which is configured to convey a recording sheet to the fixing device when the fixing device is at an appropriate temperature and to change supply timing of the recording sheet in accordance with the temperature of the fixing device so as to shorten time up to start of image formation. 
     For example, there is known an image forming apparatus having a control device configured to determine supply timing of the recording sheet. The control device is configured to set supply timing of a first recording sheet to a first timing on condition that a temperature gradient of the fixing device during predetermined time after a printing command is received is greater than a threshold value, and to set the supply timing of the first recording sheet to a second timing later than the first timing on condition that the temperature gradient is equal to or less than the threshold value. Also, the control device changes the threshold value to a smaller value when the temperature of the fixing device at the time of receiving the printing command is higher. 
     In the meantime, the time that is to be consumed until the fixing device at a cold state is heated to a fixable state changes depending on an output of a heater configured to heat the fixing device. For example, when a control of increasing the output of the heater is carried out at a state where the fixing device is cold, the heat is more likely to be transferred to the entire fixing device as compared to when a control of decreasing the output of the heater is carried out. As a result, the time that is to be consumed after the fixing device is heated until the fixing device becomes at a fixable state is shortened. For this reason, even when the temperature gradient of the fixing device during a predetermined time after the printing command is received is less than the threshold value, there are cases when the first recording sheet can be supplied at the first timing if the output of the heater is high. 
     SUMMARY 
     Aspects of the present disclosure provide an image forming apparatus capable of promptly starting supply of a recording sheet when a fixing device becomes at a fixable state while maintaining a fixing quality, a storage medium storing a program that is to be executed by a computer configured to control the image forming apparatus, and a method for controlling an image forming apparatus. 
     According to an aspect of the present disclosure, there is provided. An image forming apparatus including: a fixing device including a heater and a temperature sensor and configured to heat-fix developer on a recording sheet at a fixing region; a feeding roller configured to feed the recording sheet to the fixing device; and a controller, wherein the controller is configured to: turn on the heater when the controller receives a printing command; determine whether a temperature detected by the temperature sensor is equal to or greater than a first threshold temperature or smaller than the first threshold temperature when the controller receives the printing command; cause the feeding roller to feed the recording sheet at a first timing, in a case where the controller determines that the temperature detected by the temperature sensor is equal to or greater than the first threshold temperature; cause the feeding roller to feed the recording sheet at a second timing later than the first timing, in a case where the controller determines that the temperature detected by the temperature sensor is smaller than the first threshold temperature; and selectively set the first threshold temperature to a first value or a second value smaller than the first value when the controller receives the printing command. 
     According to another aspect of the present disclosure, there is provided a method for controlling an image forming apparatus including a fixing device including a heater and a temperature sensor and configured to heat-fix developer on a recording sheet at a fixing region and a feeding roller configured to feed the recording sheet to the fixing device, the method including: turning on the heater when a printing command is received; determining whether a temperature detected by the temperature sensor is equal to or greater than a first threshold temperature or smaller than the first threshold temperature when the printing command is received; causing the feeding roller to feed the recording sheet at a first timing, in a case where the controller determines that the temperature detected by the temperature sensor is equal to or greater than the first threshold temperature; causing the feeding roller to feed the recording sheet at a second timing later than the first timing, in a case where the controller determines that the temperature detected by the temperature sensor is smaller than the first threshold temperature; and selectively setting the first threshold temperature to a first value or a second value smaller than the first value when the printing command is received. 
     According to another aspect of the present disclosure, there is provided a non-transitory computer readable storage medium storing a program, when executed by a computer, cause an image forming apparatus to perform operations, the image forming apparatus including a fixing device including a heater and a temperature sensor and configured to heat-fix developer on a recording sheet at a fixing region and a feeding roller configured to feed the recording sheet to the fixing device, the operations including: turning on the heater when a printing command is received; determining whether a temperature detected by the temperature sensor is equal to or greater than a first threshold temperature or smaller than the first threshold temperature when the printing command is received; causing the feeding roller to feed the recording sheet at a first timing, in a case where the controller determines that the temperature detected by the temperature sensor is equal to or greater than the first threshold temperature; causing the feeding roller to feed the recording sheet at a second timing later than the first timing, in a case where the controller determines that the temperature detected by the temperature sensor is smaller than the first threshold temperature; and selectively setting the first threshold temperature to a first value or a second value smaller than the first value when the printing command is received. 
     Accordingly, by selectively setting the first threshold temperature to a first value or a second value smaller than the first value, it is possible to promptly start the supply of the recording sheet when the fixing device becomes at a fixable state while maintaining the fixing quality. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic configuration of a laser printer according to an illustrative embodiment of the disclosure: 
         FIG. 2  is a sectional view of a fixing device: 
         FIG. 3  is a perspective view of the fixing device; 
         FIG. 4  is a block diagram depicting respective sensors, a control unit, a driving source, a heater and a pickup roller; 
         FIG. 5  is a flowchart depicting a control operation of the control unit; 
         FIG. 6  is a flowchart depicting a control operation of mode selection control; 
         FIG. 7  is a timing chart depicting a difference of sheet feeding timings depending on positions of a pressing plate; 
         FIG. 8  is a timing chart depicting a difference of the sheet feeding timings depending on humidity; and 
         FIG. 9  is a timing chart depicting a difference of the sheet feeding timings depending on states of a heater. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an illustrative embodiment of the disclosure will be described in detail with reference to the drawings. In the following descriptions, a schematic configuration of a laser printer  1 , which is an example of the image forming apparatus according to an illustrative embodiment of the disclosure, will be first described and features of the disclosure will be then described. 
     As shown in  FIG. 1 , the laser printer  1  mainly has, in a main body housing  2 , a feeder unit  3  configured to supply a sheet P, which is an example of the recording sheet, an exposure device  4 , a process cartridge  5  configured to form a toner image (developer image) on the sheet P, a fixing device  100  configured to heat-fix the toner image transferred to the sheet P, a driving source  8 , a humidity sensor  9  and a control unit  10 . 
     Meanwhile, in the following descriptions, the directions are described on the basis of a user who uses the laser printer. That is, the right side of  FIG. 1  is referred to as ‘front,’ the left side is referred to as ‘rear,’ the front side is referred to as ‘left’ and the inner side is referred to as ‘right.’ Also, the upper and lower directions of  FIG. 1  are referred to as ‘upper-lower.’ 
     The feeder unit  3  is provided at a lower part in the main body housing  2 , and mainly has a sheet feeding tray  31  which accommodates therein the sheet P, a pressing plate  32  which is an example of the support plate, a pickup roller  33  which is an example of the feeding roller, a sheet feeding pad  34 , paper dust pickup rollers  35 ,  36  and registration rollers  37 . 
     The pickup roller  33  is disposed above a front end portion of the sheet feeding tray  31 . The pickup roller  33  is configured to rotate with being in contact with the sheet P accommodated in the sheet feeding tray  31 , thereby supplying the sheet P to the process cartridge  5 . 
     The pressing plate  32  is provided for the sheet feeding tray  31  and is configured to support the sheet P accommodated in the sheet feeding tray  31 . The pressing plate  32  is configured to bring a front end portion of the sheet P accommodated in the sheet feeding tray  31  close to the pickup roller  33 . The pressing plate  32  is configured to rotate about a rear end portion, thereby moving between a spaced position (refer to the broken line), which is an example of the first position located at the lowest part and distant from the pickup roller  33 , and a sheet feeding position (refer to the solid line), which is an example of the second position which the pressing plate reaches as it is rotated upward from the spaced position and at which the pressing plate is closer to the pickup roller  33  than at the spaced position and the sheet P is in contact with the pickup roller  33 . In the meantime, the sheet feeding position is different depending on the number of the sheets P accommodated in the sheet feeding tray  31 . 
     Also, the laser printer  1  has a pressing plate position sensor  32 A configured to detect whether the pressing plate  32  is located at the spaced position or the sheet feeding position. For example, when the pressing plate  32  is located at a position above or below a predetermined position, the pressing plate position sensor  32 A outputs different signals. Thereby, it is possible to detect whether the pressing plate  32  is located at the spaced position below the predetermined position or at the sheet feeding position above the predetermined position. 
     In the feeder unit  3 , the sheets P in the sheet feeding tray  31  are inclined toward the pickup roller  33  by the pressing plate  32  and are separated one by one by the pickup roller  33  and the sheet feeding pad  34 , which then passes through the paper dust pickup rollers  35 ,  36  and the registration rollers  37  and is conveyed toward the process cartridge  5 . 
     The exposure device  4  is disposed at an upper part in the main body housing  2 , and mainly has a laser light emitting unit (not shown), a polygon mirror  41  configured to rotate, lenses  42 ,  43 , and reflectors  44 ,  45 ,  46 . In the exposure device  4 , a laser light (refer to the dashed-dotted line) emitted from the laser light emitting unit on the basis of image data is reflected on or passes through the polygon mirror  41 , the lens  42 , the reflectors  44 ,  45 , the lens  43  and the reflector  46  in corresponding order and is then scanned on a surface of a photosensitive drum  61  at high speed. 
     The process cartridge  5  is disposed below the exposure device  4 , and is configured to be detachably mounted to the main body housing  2  through an opening that is to be formed when a front cover  21  provided for the main body housing  2  is opened. The process cartridge  5  has a drum unit  6  and a developing unit  7 . 
     The drum unit  6  mainly has a photosensitive drum  61 , a charger  62  and a transfer roller  63 . Also, the developing unit  7  is configured to be detachably mounted to the drum unit  6 , and mainly has a developing roller  71 , a supply roller  72 , a layer thickness regulation blade  73 , a toner accommodation unit  74  configured to accommodate therein toner (developer), and an agitator  75  configured to stir the toner in the toner accommodation unit  74  and to supply the same to the supply roller  72 . 
     In the process cartridge  5 , a surface of the photosensitive drum  61  is uniformly charged by the charger  62  and is then exposed by the high-speed scanning of the laser light from the exposure device  4 , so that an electrostatic latent image based on image data is formed on the photosensitive drum  61 . Also, the toner in the toner accommodation unit  74  is supplied to the developing roller  71  via the supply roller  72 , is introduced between the developing roller  71  and the layer thickness regulation blade  73  and is carried on the developing roller  71  as a thin layer having a predetermined thickness. 
     The toner carried on the developing roller  71  is supplied from the developing roller  71  to the electrostatic latent image formed on the photosensitive drum  61 . Thereby, the electrostatic latent image becomes visible and a toner image is thus formed on the photosensitive drum  61 . Thereafter, the sheet P is conveyed between the photosensitive drum  61  and the transfer roller  63 , so that the toner image on the photosensitive drum  61  is transferred to the sheet P. 
     The fixing device  100  is provided at the rear of the process cartridge  5 . The fixing device  100  is configured to convey and heat the sheet P delivered from the process cartridge  5 , thereby heat-fixing the toner image transferred to the sheet P on the sheet P. The sheet P of which the toner image has been heat-fixed by the fixing device  100  is discharged onto a sheet discharge tray  22  by conveying rollers  23 ,  24 . 
     As shown in  FIGS. 2 and 3 , the fixing device  110  mainly has a fixing belt  110 , a heater  120 , a nip plate  130 , a reflection plate  140 , a pressing roller  150 , a stay  160 , a first thermistor  171 , which is an example of the temperature sensor, a second thermistor  172  and a thermostat  180 . 
     The fixing belt  110  is an endless (cylindrical) member having heat resistance and flexibility, and both end portions in a width direction are rotation-guided by guide members (not shown). 
     The heater  120  is a halogen lamp, for example, and is configured to heat the fixing belt  110  (nip portion N) via the nip plate  130 , thereby heating the toner on the sheet P. The heater  120  is disposed at a predetermined interval from inner surfaces of the fixing belt  110  and the nip plate  130 , inside the fixing belt  110 . 
     The nip plate  130  is a plate-shaped member configured to be applied with a pressing force from the pressing roller  150  and to transmit radiation heat from the heater  120  to the toner on the sheet P via the fixing belt  110 , and is disposed so that a lower surface thereof is in sliding contact with the inner surface of the cylindrical fixing belt  110 . 
     The nip plate  130  is made of an aluminum plate having a greater heat conductivity than the stay  160  made of steel (which will be described later), for example, and mainly has a substantially flat plate-shaped base part  131  and protrusions  132 . 
     The protrusion  132  is formed to protrude rearward from a rear end portion  131 R in a conveying direction of the base part  131  along the conveying direction. As shown in  FIG. 3 , the two protrusions  132  are provided in the vicinities of a right end and a center of the rear end portion  131 R of the base part  131 , respectively. 
     As shown in  FIG. 2 , the reflection plate  140  is a member configured to reflect the radiation heat from the heater  120  toward the nip plate  130  (an inner surface of the base part  131 ), and is disposed at a predetermined interval from the heater  120  inside the fixing belt  110  so as to surround the heater  120 . 
     The reflection plate  140  is formed by bending an aluminum plate having great reflectivity of infrared and far infrared rays into a substantial U shape, as seen from a sectional view, for example. More specifically, the reflection plate  140  mainly has a reflection part  141  having a bent shape (a substantial U shape, as seen from a sectional view) and flange portions  142  extending from both end portions of the reflection part  141  along the conveying direction. 
     The pressing roller  150  is a member configured to sandwich the fixing belt  110  between the pressing roller and the nip plate  130  and to form a nip portion N between the pressing roller and the fixing belt  110 , and is disposed below the nip plate  130 . In the illustrative embodiment, the nip plate  130  is urged toward the pressing roller  150  so as to form the nip portion N. The pressing roller  150  is configured to rotate with sandwiching the fixing belt  110  between the pressing roller and the nip plate  130 , thereby rotating together with the fixing belt  110  to convey rearward the sheet P. In the meantime, a configuration where the pressing roller  150  is urged toward the nip plate  130  so as to form the nip portion N may also be adopted. 
     The pressing roller  150  is configured to rotate, thereby rotating the fixing belt  110  by a frictional force with the fixing belt  110  (or the sheet P). The sheet P having the toner image transferred thereto is conveyed between the pressing roller  150  and the heated fixing belt  110  (the nip portion N), so that the toner image (toner) is heat-fixed. 
     The stay  160  is a member configured to support both end portions in a front-rear direction of the base part  131  of the nip plate  130  and to secure rigidity of the nip plate  130 , has a substantial U shape conforming to the outer surface shape of the reflection part  141  of the reflection plate  140 , as seen from a sectional view, and is disposed to cover the reflection plate  140 . The stay  160  is formed by bending a steel plate having relatively high rigidity into a substantial U shape, as seen from a sectional view, for example. The stay  160  is configured to sandwich the flange portions  142  of the reflection plate  140  between the stay and the nip plate  130 . 
     As shown in  FIG. 3 , a rear wall  160 R of the stay  160  has two notches  161  for disposing the first thermistor  171  and the second thermistor  172 . More specifically, the notches  161  are respectively formed to have a gap so as not to contact the first thermistor  171  and the second thermistor  172  at positions corresponding to the two protrusions  132  of the nip plate  130 . 
     The first thermistor  171  and the second thermistor  172  are temperature sensors, and are disposed to detect a temperature of the fixing device  100 , in the illustrative embodiment, a temperature of the nip plate  130 . The first thermistor  171  is disposed in the vicinity of the center in the left-right direction of the nip plate  130  and is configured to detect a temperature of the nip plate  130  within a range in which the sheet P is to be conveyed. The second thermistor  172  is disposed at the right end portion of the nip plate  130 , and is configured to detect a temperature beyond a range in which a small sheet P (for example, a postcard or an A6 sheet) is to be conveyed. 
     Also, as shown in  FIGS. 2 and 3 , the respective thermistors  171 ,  172  are fixed to the rear wall  160 R of the stay  160  by screws  179  and are disposed to face upper surfaces (opposite surfaces to the sliding contact surfaces with the fixing belt  110 ) of the protrusions  132  of the nip plate  130 , inside the fixing belt  110 . The respective thermistors  171 ,  172  are disposed so that temperature detection surfaces  171 A are in contact with the upper surfaces of the protrusion  132 . 
     Also, the respective thermistors  171 ,  172  are disposed outside the reflection plate  140  with respect to the conveying direction. More specifically, the respective thermistors  171 ,  172  are disposed at a downstream side (rear side) of the reflection plate  140  with respect to the conveying direction, at an outer side of the nip portion N in the conveying direction. Also, the respective thermistors  171 ,  172  are disposed with a gap from the reflection plate  140  so that they are not in contact with an outer surface of the reflection plate  140 . 
     The thermostat  180  is a temperature detection element using bimetal or the like, and is disposed to detect a temperature of the reflection plate  140 . Specifically, the thermostat  180  is fixed to the stay  160  inside the fixing belt  110  (refer to  FIG. 3 ), and is disposed above the reflection plate  140  so that a temperature detection surface  181  faces the reflection plate  140 . 
     The thermostat  180  is provided on a circuit configured to feed power to the heater  120  and cuts off energization to the heater  120  when a predetermined temperature or higher is detected. Thereby, it is possible to prevent the temperature of the fixing device  100  from excessively increasing. 
     As shown in  FIG. 1 , the driving source  8  is configured to drive the pressing plate  32 , the fixing device  100  and the developing unit  7 . In the illustrative embodiment, the driving source  8  is configured to drive the pressing roller  150  of the fixing device  100 . The driving source  8  includes a motor (not shown). Also, the laser printer  1  has a plurality of gear trains (not shown) configured to couple the developing unit  7  and the driving source  8 . A driving force is transmitted to the developing unit  7  from the driving source  8 , so that the developing roller  71 , the supply roller  72  and the agitator  75  are rotated. That is, the driving source  8  is configured to drive the developing roller  71 , the supply roller  72  and the agitator  75 . 
     The humidity sensor  9  is a sensor configured to detect humidity in the main body housing  2 , and is provided at an appropriate position in the main body housing  2 . 
     As shown in  FIG. 4 , the control unit  10  has a CPU, a ROM, a RAM and the like, and is configured to control the driving source  8 , the heater  120  and the pickup roller  33  on the basis of a program prepared in advance and information acquired by the pressing plate position sensor  32 A, the first thermistor  171  and the humidity sensor  9 , thereby starting supply of the sheet P at predetermined timing when the fixing device  100  is at an appropriate temperature. 
     When a printing command is received, the control unit  10  turns on the heater  120  to start heating of the fixing device  100 , sets a heater control instruction value so that the temperature of the fixing device  100  becomes a target temperature, based on the temperature T detected by the first thermistor  171 , and adjusts an output of the heater  120 . The heater control instruction value is a duty ratio indicative of an energization ratio per time, for example. The higher the heater control instruction value is, the output of the heater  120  becomes higher. 
     Also, the control unit  10  is configured to selectively execute a first output control in which the output of the heater  120  becomes a first output and a second output control in which the output of the heater  120  becomes a second output higher than the first output. Specifically, when the pressing plate  32  is located at the sheet feeding position at the time of receiving of the printing command, the control unit  10  executes the first output control in which an output of the driving source  8  is set to a third output M 3  less than a maximum speed. When the pressing plate  32  is located at the spaced position at the time of receiving the printing command, the control unit  10  executes the second output control in which the output of the driving source  8  is set to a fourth output M 4  (for example, maximum speed) greater than the third output M 3  so as to promptly move the pressing plate  32  to the sheet feeding position. That is, the control unit  10  is configured to select the first output control and the second output control in accordance with the position of the pressing plate  32  at the time of receiving the printing command (selection processing). 
     When the first output control and the second output control are executed in this way and the second output control is thus carried out, the pressing roller  150  is rotated at higher speed, as compared to when the first output control is executed. When the pressing roller  150  is rotated, the heat of the nip plate  130  can be easily transmitted to the entire fixing device  100 . Therefore, when the second output control in which the pressing roller  150  is rotated at higher speed is carried out, the heat of the nip plate  130  can be more easily transmitted, as compared to when the first output control is executed. As a result, the heater control instruction value is kept large, so that the output of the heater  120  increases. 
     Also, after the first output control and the second output control, when starting image formation, i.e., starting the supply of the sheet P, the control unit  10  sets the output of the driving source  8  to the fourth output M 4 . 
     At predetermined timing after starting the heating of the fixing device  100 , in the illustrative embodiment, after second predetermined time B 2  elapses from the turn-on of the heater  120 , when a detected temperature T B2  of the first thermistor  171  is equal to or greater than a first threshold value TH 1 , the control unit  10  executes a first supply control (first supply control execution processing) of supplying the first sheet P at first timing by the pickup roller  33 . In the meantime, the timing at which the control unit  10  compares the temperature T detected by the first thermistor  171  and the first threshold value TH 1  may be after predetermined time elapses from the reception of the printing command or after predetermined time elapses from the driving of the driving source  8 . When the detected temperature T B2  of the first thermistor  171  is less than the first threshold value TH 1  after second predetermined time B 2  elapses from the turn-on of the heater  120 , the control unit  10  executes a second supply control (second supply control execution processing) of supplying the first sheet P at second timing later than the first timing by the pickup roller  33 . 
     Specifically, the first timing is a point of time at which standby time t 0  elapses after the heater  120  is turned on. That is, in the first supply control, when the standby time t 0  elapses after starting the heating of the fixing device  110 , the control unit  10  rotates the pickup roller  33  to supply the first sheet P. In the meantime, the first timing may be a point of time at which the standby time t 0  elapses after the printing command is received or may be a point of time at which the standby time t 0  elapses after the driving source  8  is driven. Also, the second timing is a point of time at which the temperature T detected by the first thermistor  171  reaches a predetermined temperature TH 3  after the heater  120  is turned on. That is, in the second supply control, when the temperature T detected by the first thermistor  171  reaches the predetermined temperature TH 3  after starting the heating of the fixing device  100 , the control unit  10  rotates the pickup roller  33  to supply the first sheet P. In the meantime, the standby time t 0  is set to the shortest time during which the fixing device  100  becomes at an appropriate temperature until the supplied first sheet P reaches the fixing device  100 . Also, the predetermined temperature TH 3  is set to the lowest temperature at which the fixing device  100  becomes at an appropriate temperature until the supplied first sheet P reaches the fixing device  100 . 
     Also, in the second supply control, after the second predetermined time B 2  from the turn-on of the heater  120 , when the detected temperature T B2  of the first thermistor  171  is less than a second threshold value TH 2  smaller than the first threshold value TH 1 , the control unit  10  sets the predetermined temperature TH 3  to a lower value, as compared to when the detected temperature T B2  of the first thermistor  171  is equal to or greater than the second threshold value TH 2 . Specifically, after the second predetermined time B 2  from the turn-on of the heater  120 , when the detected temperature T B2  of the first thermistor  171  is equal to or greater than the second threshold value TH 2 , the control unit  10  sets the predetermined temperature TH 3  to a first predetermined temperature T 1 , and when the detected temperature T B2  of the first thermistor  171  is less than the second threshold value TH 2 , the control unit  10  sets the predetermined temperature TH 3  to a second predetermined temperature T 2  lower than the first predetermined temperature T 1 . 
     In the meantime, the control unit  10  is configured to control a motor, which is configured to drive the pickup roller  33  provided separately from the driving source  8 , thereby controlling rotation/stop of the pickup roller  33 . The control of the rotation/stop of the pickup roller  33  by the control unit  10  is not limited thereto. For example, the control unit may be configured to control a solenoid actuator configured to vertically drive the pickup roller  33 , or the like. Also, the laser printer  1  may have a driving transmission mechanism capable of switching between a connection state where the driving force of the driving source  8  can be transmitted to the pickup roller  33  and a disconnection state where the driving force of the driving source  8  is cut off, and the control unit  10  may be configured to control the driving transmission mechanism, thereby controlling the rotation/stop of the pickup roller  33 . 
     The control unit  10  is configured to set the first threshold value TH 1 , which is to be used when executing the second output control, to a value less than the first threshold value TH 1 , which is to be used when executing the first output control (first threshold value setting processing). 
     Also, the control unit  10  is configured to set the first threshold value TH 1 , which is to be used when a humidity detected by the humidity sensor  9  is equal to or greater than a predetermined humidity, to a value greater than the first threshold value TH 1 , which is to be used when the humidity detected by the humidity sensor  9  is less than the predetermined humidity. 
     When a first temperature detected by the first thermistor  171  after first predetermined time B 1  from the turn-on of the heater  120 , i.e., from the start of the heating of the fixing device  100  is higher than a second temperature detected by the first thermistor  171  after second predetermined time B 2  longer than the first predetermined time B 1  from the start of the heating of the fixing device  100 , the control unit  10  sets the first threshold value TH 1  to a value smaller than the first threshold value TH 1  that is to be used when the first temperature is equal to or lower than the second temperature. More specifically, when the maximum value Tmax of the temperature T detected by the first thermistor  171  after the heating of the fixing device  100  starts until the second predetermined time B 2  elapses is greater than the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the start of the heating of the fixing device  100 , the control unit  10  sets the first threshold value TH to a value smaller than the first threshold value TH 1  that is to be used when the maximum value Tmax of the temperature T detected by the first thermistor  171  after the heating of the fixing device  110  starts until the second predetermined time B 2  elapses is the same as the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the start of the heating of the fixing device  100 . 
     Herein, a difference of the temperature increase rate of the fixing device  100  is described. The timing at which the temperature T detected by the first thermistor  171  after the heater  120  is turned on until the second predetermined time B 2  elapses becomes the maximum value Tmax is different depending on the temperature of the fixing device  100  or a heat generation state of the heater  120  upon the turn-on of the heater  120 . For example, the heat generation state of the heater  120  changes due to manufacturing non-uniformity of the output of the heater  120  or non-uniformity of a power supply voltage. For this reason, when the heat generation state of the heater  120  is favorable, the temperature T detected by the first thermistor  171  is more likely to increase, as compared to when the heat generation state of the heater  120  is poor, as shown in  FIG. 9 . In a case where the heat generation state of the heater  120  is favorable and the laser printer  1  is used under environments of room temperature or higher, when the first predetermined time B 1  elapses before the second predetermined time B 2  elapses after the heater  120  is turned on, the temperature T detected by the first thermistor  171  becomes the maximum value Tmax. When the temperature T detected by the first thermistor  171  becomes the maximum value Tmax, the control unit  10  decreases the heater control instruction value. Thereby, at the point of time at which the second predetermined time B 2  elapses after the heater  120  is turned on, the detected temperature T B2  becomes less than the maximum value Tmax. On the other hand, when the heat generation state of the heater  120  is poor or the laser printer  1  is used under low temperature environments, the control unit  10  keeps the heater control instruction value high. At the point of time at which the second predetermined time B 2  elapses after the heater  120  is turned on, the detected temperature T B2  of the first thermistor  171  becomes the maximum value Tmax of the temperature T detected by the first thermistor  171  after the heater  120  is turned on until the second predetermined time B 2  elapses. That is, when the temperature T detected by the first thermistor  171  becomes the maximum value Tmax before the second predetermined time B 2  elapses after the heater  120  is turned on, it is possible to set the first threshold value TH 1  to the slight small value because the heat generation state of the heater  120  is favorable and the fixing device  100  is sufficiently warmed. 
     Like the first threshold value TH 1  as described above, the control unit  10  is configured to set the second threshold value TH 2  depending on the respective conditions. That is, the control unit  10  is configured to set the second threshold value TH 2 , which is to be used when executing the second output control, to a value smaller than the second threshold value TH 2 , which is to be used when executing the first output control. Also, the control unit  10  is configured to set the second threshold value TH 2 , which is to be used when the humidity detected by the humidity sensor  9  is equal to or greater than the predetermined humidity, to a value greater than the second threshold value TH 2 , which is to be used when the humidity detected by the humidity sensor  9  is less than the predetermined humidity. When the first temperature detected by the first thermistor  171  after the first predetermined time B 1  from the turn-on of the heater  120  is higher than the second temperature detected by the first thermistor  171  after the second predetermined time B 2  longer than the first predetermined time B 1  from the start of the heating of the fixing device  100 , the control unit  10  sets the second threshold value TH 2  to a value smaller than the second threshold value TH 2 , which is to be used when the first temperature is equal to or lower than the second temperature. 
     In order to set the first threshold value TH 1  and the second threshold value TH 2  as described above, the control unit  10  is configured to store therein a map, which indicates values of the first threshold value TH 1  and the second threshold value TH 2  corresponding to combinations of respective states (a position of the pressing plate  32  at the time of receiving the printing command, the humidity, and the difference of the temperature increase rate of the fixing device  100 ). The control unit  10  is configured to acquire the first threshold value TH 1  and the second threshold value TH 2  by referring to the map. 
     Subsequently, a specific control operation of the control unit  10  is described. 
     As shown in  FIG. 5 , when the printing command is received (S 1 ), the control unit  10  determines whether the pressing plate  32  is located at the spaced position (S 2 ). 
     When it is determined in step S 2  that the pressing plate  32  is located at the spaced position (S 2 , Yes), the control unit  10  sets a pressing plate flag F P  to 1 (S 21 ). On the other hand, when it is determined in step S 2  that the pressing plate  32  is not located at the spaced position (S 2 , No), the control unit  10  sets the pressing plate flag F P  to zero (0) (S 22 ). 
     When the pressing plate flag F P  is set in step S 21  or step S 22 , the control unit  10  determines whether the humidity is equal to or greater than the predetermined humidity from a detection result of the humidity sensor  9  (S 3 ). 
     When it is determined in step S 3  that the humidity is equal to or greater than the predetermined humidity (S 3 , Yes), the control unit  10  sets a humidity flag F F  to 1 (S 31 ). On the other hand, when it is determined in step S 3  that the humidity is less than the predetermined humidity (S 3 , No), the control unit  10  sets the humidity flag F F  to zero (0) (S 32 ). 
     When the humidity flag F F  is set in step S 31  or step S 32 , the control unit  10  determines whether the maximum value Tmax of the temperature T detected by the first thermistor  171  is the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the start of the heating of the fixing device  100  (S 4 ). 
     When it is determined in step S 4  that the maximum value Tmax of the temperature T detected by the first thermistor  171  is the detected temperature T B2  after the second predetermined time B 2  from the start of the heating of the fixing device  100  (S 4 , Yes), the control unit  10  sets a temperature flag F T  to 1 (S 41 ). On the other hand, when it is determined in step S 4  that the maximum value Tmax of the temperature T detected by the first thermistor  171  is not the detected temperature T B2  after the second predetermined time B 2  from the start of the heating of the fixing device  100  (S 4 , No), the control unit  10  sets the temperature flag F T  to zero (0) (S 42 ). 
     When the temperature flag F T  is set in step S 41  or step S 42 , the control unit  10  refers to the map to set the values of the first threshold value TH 1  and the second threshold value TH 2  based on the pressing plate flag F P , the humidity flag F F  and the temperature flag F T  (S 5 ). Then, the control unit  10  executes mode selection (S 10 ), and ends the control. 
     As shown in  FIG. 6 , when the control unit  10  starts the mode selection, the control unit  10  determines whether the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the start of the heating of the fixing device  100  is equal to or greater than the first threshold value TH 1  (S 11 ). 
     When it is determined in step S 11  that the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the start of the heating of the fixing device  100  is equal to or greater than the first threshold value TH 1  (S 11 , Yes), the control unit  10  determines whether the standby time t 0  has elapsed from the start of the heating of the fixing device  100  (S 12 ). When it is determined in step S 12  that the standby time to has not elapsed from the start of the heating of the fixing device  100  (S 12 , No), the control unit  10  stands by until the standby time t 0  elapses from the start of the heating of the fixing device  100 . When it is determined in step S 12  that the standby time t 0  has elapsed from the start of the heating of the fixing device  100  (S 12 , Yes), the control unit  10  rotates the pickup roller  33  to pick up the first sheet P (S 13 ). 
     When it is determined in step S 11  that the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the start of the heating of the fixing device  100  is not equal to or greater than the first threshold value TH 1  (S 11 , No), the control unit  10  determines whether the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the start of the heating of the fixing device  100  is equal to or greater than the second threshold value TH 2  (S 14 ). 
     When it is determined in step S 14  that the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the start of the heating of the fixing device  100  is equal to or greater than the second threshold value TH 2  (S 14 , Yes), the control unit  10  sets the predetermined temperature TH 3  to the first predetermined temperature T 1  (S 141 ). On the other hand, when it is determined in step S 14  that the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the start of the heating of the fixing device  100  is not equal to or greater than the second threshold value TH 2  (S 14 , No), the control unit  10  sets the predetermined temperature TH 3  to the second predetermined temperature T 2  lower than the first predetermined temperature T 1  (S 142 ). 
     When the value of the predetermined temperature TH 3  is set in step S 141  or step S 142 , the control unit  10  determines whether the temperature T detected by the first thermistor  171  is equal to or greater than the predetermined temperature TH 3  (S 15 ). 
     When it is determined in step S 15  that the temperature T detected by the first thermistor  171  is not equal to or greater than the predetermined temperature TH 3  (S 15 , No), the control unit  10  stands by until the temperature T detected by the first thermistor  171  becomes equal to or greater than the predetermined temperature TH 3 . When it is determined in step S 15  that the temperature T detected by the first thermistor  171  is equal to or greater than the predetermined temperature TH 3  (S 15 , Yes), the control unit  10  proceeds to step S 13  and rotates the pickup roller  33  to pick up the first sheet P. 
     After picking up the first sheet P in step S 13 , the control unit  10  ends the mode selection. 
     Subsequently, operations of the laser printer  1  are described. 
     First, a case where the position of the pressing plate  32  is different is described with reference to  FIG. 7 . Meanwhile, regarding the respective conditions shown in  FIG. 7 , it is assumed that the humidity is the same and the heat generation state of the heater  120  is poor. 
     When the pressing plate  32  is located at the spaced position (refer to the solid line) at the time that the control unit  10  receives the printing command, the control unit  10  sets the output of the driving source  8  to the fourth output M 4  and turns on the heater  120  (time t 1 ). Also, when the pressing plate  32  is located at the sheet feeding position (refer to the broken line) at the time that the control unit  10  receives the printing command, the control unit  10  sets the output of the driving source  8  to the third output M 3  and turns on the heater  120  (time t 1 ). When the heater  120  is turned on in this way, the temperature T detected by the first thermistor  171  starts to increase. 
     After the heater  120  is turned on, the control unit  10  turns on the driving source  8  with the set output (time t 2 ). Thereby, the pressing roller  150  of the fixing device  100  rotates. When the pressing plate  32  is located at the spaced position at the time of receiving the printing command, since the output of the driving source  8  is higher, as compared to when the pressing plate  32  is located at the sheet feeding position, the pressing roller  150  rotates fast, so that the heat of the nip plate  130  is promptly spread over the entire fixing device  100 . Thereby, the temperature T detected by the first thermistor  171  more gently increases when the pressing plate  32  is located at the spaced position, as compared to when the pressing plate  32  is located at the sheet feeding position. For this reason, the control unit  10  keeps the heater control instruction value higher when the pressing plate  32  is located at the spaced position, as compared to when the pressing plate  32  is located at the sheet feeding position, so that the output of the heater  120  increases. 
     When the second predetermined time B 2  elapses (time t 3 ) after the heater  120  is turned on, the control unit  10  sets the first threshold value TH 1  to a temperature A 11  and the second threshold value TH 2  to a temperature A 12  if the pressing plate  32  is located at the spaced position at the time of receiving the printing command. On the other hand, if the pressing plate  32  is located at the sheet feeding position at the time of receiving the printing command, the control unit  10  sets the first threshold value TH 1  to a temperature A 21  higher than the temperature A 11  and the second threshold value TH 2  to a temperature A 22  higher than the temperature A 12 . 
     At this time, if the detected temperature TB 2  of the first thermistor  171  is equal to or greater than the first threshold value TH 1 , when the standby time t 0  elapses (time t 4 ) from the turn-on of the heater  120 , the pickup roller  33  rotates to pick up the first sheet P (refer to the bold solid line and the bold dashed-dotted line). 
     Herein, when the pressing plate  32  is located at the spaced position at the time of receiving the printing command, the pressing roller  150  rotates fast and the output of the heater  120  is high. Therefore, even when the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the turn-on of the heater  120  is a value smaller than the temperature A 21 , there is a possibility that the fixing device  100  becomes at an appropriate temperature at which the sheet feeding starts after the standby time to elapses from the turn-on of the heater  120 . For this reason, when the pressing plate  32  is located at the spaced position at the time of receiving the printing command, the first threshold value TH 1  is set to the temperature A 11  lower than the temperature A 12 , which is the first threshold value TH to be used when the pressing plate  32  is located the sheet feeding position, so that it is possible to promptly start to feed the sheet at appropriate timing at a condition by which the fixing device  100  becomes at an appropriate temperature after the standby time t 0  from the turn-on of the heater  120 . 
     However, as shown with the thin solid line and dashed-dotted line in  FIG. 7 , when the laser printer  1  is under low temperature environments and the temperature T detected by the first thermistor  171  at the time of receiving the printing command is low, the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the turn-on of the heater  120  becomes smaller than the first threshold value TH 1  and the second threshold value TH 2 . In this case, when the temperature T detected by the first thermistor  171  becomes equal to or higher than the predetermined temperature TH 3 , the pickup roller  33  rotates to pick up the first sheet P (time t 5  and time t 6 ). Like this, if the temperature of the fixing device  100  at the time of receiving the printing command is low, since the sheet feeding starts when the temperature of the fixing device  100  sufficiently increases, it is possible to promptly start the sheet feeding when the fixing device  100  becomes at an appropriate temperature. 
     Subsequently, a case where the humidity is different is described with reference to  FIG. 8 . Meanwhile, regarding the respective conditions shown in  FIG. 8 , it is assumed that the pressing plate  32  is located at the spaced position at the time of receiving the printing command and the heat generation state of the heater  120  is poor. 
     When the printing command is received and the heater  120  is thus turned on (time t 1 ), the temperature T detected by the first thermistor  171  starts to increase. At this time, when the humidity is high (refer to the dashed-dotted line), since the sheet P absorbs the moisture, the temperature T detected by the first thermistor  171  more gently increases, as compared to when the humidity is low (refer to the solid line). Thereby, when the humidity is high, since the control unit  10  keeps the heater control instruction value large, the output of the heater  120  is higher when the humidity is high, as compared to when the humidity is low. 
     After the second predetermined time B 2  elapses from the turn-on of the heater  120 , when the humidity is less than a predetermined humidity (low humidity), the control unit  10  sets the first threshold value TH 1  to a temperature A 31  and the second threshold value TH 2  to a temperature A 32 . Also, after the second predetermined time B 2  elapses from the turn-on of the heater  120 , when the humidity is equal to or higher than the predetermined humidity (high humidity), the control unit  10  sets the first threshold value TH 1  to a temperature A 41  higher than the temperature A 31  and the second threshold value TH 2  to a temperature A 42  higher than the temperature A 32 . 
     At this time, when the detected temperature T B2  of the first thermistor  171  is equal to or greater than the first threshold value TH 1  (refer to the bold solid line and dashed-dotted line), the pickup roller  33  rotates to pick up the first sheet P (time t 4 ) after the standby time to elapses from the turn-on of the heater  120 . 
     Herein, when the humidity is equal to or greater than the predetermined humidity, the output of the heater  120  is high. Therefore, even when the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the turn-on of the heater  120  is a value smaller than the temperature A 41 , there is a possibility that the fixing device  100  becomes at an appropriate temperature after the standby time t 0  elapses from the turn-on of the heater  120 . For this reason, when the humidity is equal to or greater than the predetermined humidity, the first threshold value TH 1  is set to the temperature A 31  lower than the temperature A 41 , which is the first threshold value TH 1  to be used when the humidity is less than the predetermined humidity, so that it is possible to promptly start to feed the sheet at appropriate timing at a condition by which the fixing device  100  becomes at an appropriate temperature after the standby time t 0  from the turn-on of the heater  120 . 
     However, as shown with the thin solid line in  FIG. 8 , when the laser printer  1  is under low humidity/low temperature environments, the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the turn-on of the heater  120  becomes lower than the temperature A 41  and higher than the temperature A 42 . In this case, the control unit  10  sets the predetermined temperature TH 3  to the first predetermined temperature T 1 . Thereby, when the temperature T detected by the first thermistor  171  becomes equal to or higher than the first predetermined temperature T 1 , the pickup roller  33  rotates to pick up the first sheet P (time t 7 ). 
     Also, as shown with the thin dashed-dotted line in  FIG. 8 , when the laser printer  1  is under high humidity/low temperature environments, if the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the turn-on of the heater  120  becomes lower than the temperature A 31  and the temperature A 32 , the control unit  10  sets the predetermined temperature TH 3  to the second predetermined temperature T 2  lower than the first predetermined temperature T 1 . Thereby, when the temperature T detected by the first thermistor  171  becomes equal to or higher than the second predetermined temperature T 2 , the pickup roller  33  rotates to pick up the first sheet P (time t 8 ). 
     Like this, if the temperature of the fixing device  100  is low at the time of receiving the printing command, since the sheet feeding starts when the temperature of the fixing device  100  sufficiently increases, it is possible to promptly start the sheet feeding when the fixing device  100  becomes at an appropriate temperature. 
     Subsequently, a case where the heat generation state of the heater  120  is different is described with reference to  FIG. 9 . Meanwhile, regarding the respective conditions shown in  FIG. 9 , it is assumed that the pressing plate  32  is located at the spaced position at the time of receiving the printing command and the humidity is the same. 
     When the printing command is received and the heater  120  is thus turned on (time t 1 ), the temperature T detected by the first thermistor  171  starts to increase. At this time, when the heat generation state of the heater  120  is poor (refer to the dashed-dotted line), the temperature T detected by the first thermistor  171  more gently increases, as compared to when the heat generation state of the heater  120  is favorable (refer to the solid line). Thereby, the output of the heater  120  is lower when the heat generation state of the heater  120  is poor, as compared to when the heat generation state of the heater  120  is favorable. Meanwhile, in  FIG. 9 , when the heat generation state of the heater  120  is favorable, it is denoted as “high heat generation,” and when the heat generation state of the heater  120  is poor, it is denoted as “low heat generation.” 
     At the time that the second predetermined time B 2  elapses (time t 3 ) after the heater  120  is turned on, when the detected temperature T B2  of the first thermistor  171  is not the maximum value Tmax of the temperature T detected by the first thermistor  171  (the heat generation state of the heater  120  is good and the temperature is a room temperature), the control unit  10  sets the first threshold value TH 1  to a temperature A 51  and the second threshold value TH 2  to a temperature A 52 . Also, at the time that the second predetermined time B 2  elapses after the heater  120  is turned on, when the detected temperature T B2  of the first thermistor  171  is the maximum value Tmax of the temperature T detected by the first thermistor  171  (when the heat generation state of the heater  120  is good and the temperature is a low temperature and when the heat generation state of the heater  120  is poor), the control unit  10  sets the first threshold value TH to a temperature A 61  higher than the temperature A 51  and the second threshold value TH 2  to a temperature A 62  higher than the temperature A 52 . 
     At this time, when the detected temperature T B2  of the first thermistor  171  is equal to or greater than the first threshold value TH 1  (refer to the bold solid line and dashed-dotted line), the pickup roller  33  rotates to pick up the first sheet P (time t 4 ) after the standby time t 0  elapses from the turn-on of the heater  120 . 
     Herein, when the heat generation state of the heater  120  is good and the laser printer  1  is used under room temperature environments (refer to the bold solid line), the temperature T detected by the first thermistor  171  after the heater  120  is turned on until the second predetermined time B 2  elapses becomes the maximum value Tmax and the fixing device  100  is sufficiently warmed. Therefore, even when the detected temperature TB 2  of the first thermistor  171  after the second predetermined time B 2  from the turn-on of the heater  120  is a value smaller than the temperature A 61 , there is a possibility that the fixing device  100  becomes at an appropriate temperature after the standby time to elapses from the turn-on of the heater  120 . For this reason, when the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the turn-on of the heater  120  is not the maximum value Tmax of the temperature T detected by the first thermistor  171 , the first threshold value TH 1  is set to the temperature A 51  lower than the temperature A 61 , which is the first threshold value TH 1  to be used when the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the turn-on of the heater  120  is the maximum value Tmax of the temperature T detected by the first thermistor  171 , so that it is possible to promptly start to feed the sheet at appropriate timing at a condition by which the fixing device  100  becomes at an appropriate temperature after the standby time t 0  from the turn-on of the heater  120 . 
     However, as shown with the thin solid line in  FIG. 7 , when the heat generation state of the heater  120  is good but the laser printer  1  is under low temperature environments, the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the turn-on of the heater  120  may become lower than the temperature A 61  and higher than the temperature A 62 . In this case, the control unit  10  sets the predetermined temperature TH 3  to the first predetermined temperature T 1 . Thereby, when the temperature T detected by the first thermistor  171  becomes equal to or higher than the first predetermined temperature T 1 , the pickup roller  33  rotates to pick up the first sheet P (time t 9 ). 
     Also, as shown with the thin dashed-dotted line in  FIG. 7 , when the heat generation state of the heater  120  is poor and the laser printer  1  is under low temperature environments, the detected temperature T B2  of the first thermistor  171  after the second predetermined time B 2  from the turn-on of the heater  120  may become lower than the temperature A 51  and the temperature A 52 . In this case, the control unit  10  sets the predetermined temperature TH 3  to the second predetermined temperature T 2  higher than the first predetermined temperature T 1 . Thereby, when the temperature T detected by the first thermistor  171  becomes equal to or higher than the second predetermined temperature T 2 , the pickup roller  33  rotates to pick up the first sheet P (time t 10 ). 
     Like this, if the temperature of the fixing device  100  is low at the time of receiving the printing command, since the sheet feeding starts when the temperature of the fixing device  100  sufficiently increases, it is possible to promptly start the sheet feeding when the fixing device  100  becomes at an appropriate temperature. 
     As described above, according to the illustrative embodiment, since the first threshold value TH 1  is changed in correspondence to the output of the heater  120 , it is possible to promptly start the sheet feeding at an appropriate timing after the printing command is received while maintaining a fixing quality. 
     Also, when the pressing plate  32  is located at the sheet feeding position, the first output control of setting the output of the driving source  8  to the third small output M 3  is enabled, so that the number of rotations of the developing roller  71 , the supply roller  72  and the agitator  75  of the developing unit  7  is reduced, as compared to the laser printer  1  configured to always perform the control by the second output control (of setting the output of the driving source  8  to the fourth output M 4 ). Thereby, it is possible to suppress the toner from being deteriorated. 
     Although the illustrative embodiment of the disclosure has been described, the disclosure is not limited to the above illustrative embodiment. The specific configuration can be appropriately changed without departing from the gist of the disclosure. 
     In the above illustrative embodiment, the control unit  10  is configured to control the rotation/stop of the pickup roller  33 , thereby controlling the supply timing of the sheet P. However, the method of controlling the supply timing of the sheet P is not limited thereto. For example, the pickup roller  33  may be provided close to or spaced from the pressing plate  32  located at the sheet feeding position and the control unit  10  may be configured to bring the pickup roller  33  close to the pressing plate  32 , to bring the pickup roller  33  into contact with the sheet P and to rotate the pickup roller  33  at the supply timing of the sheet P. Also, the control unit  10  may be configured to rotate the pickup roller  33  after receiving the printing command and to bring the pickup roller  33  close to the pressing plate  32  at the supply timing of the sheet P. 
     In the above illustrative embodiment, the pickup roller  33  has been exemplified as the feeding roller. However, the feeding roller is not limited thereto, and may be the registration rollers  37 , for example. 
     In the above illustrative embodiment, the fixing device  100  having the fixing belt  110  and the nip plate  130  has been exemplified. However, the configuration of the fixing device is not limited thereto. For example, the fixing device may have a heating roller, which is to be heated by the heater, and a pressing roller configured to sandwich and convey the sheet P between the pressing roller and the heating roller. In this case, the temperature sensor may be provided to detect a temperature of the heating roller. 
     In the above illustrative embodiment, the laser printer  1  has the pressing plate position sensor  32 A configured to detect the position of the pressing plate  32 . However, the method of determining the position of the pressing plate  32  is not limited thereto. For example, the laser printer  1  may have a pressing plate position estimation unit configured to store the accumulated number of rotations of the driving source  8  in a memory and to estimate the position of the pressing plate  32 . In the meantime, the accumulated number of rotations of the driving source  8  stored in the memory is reset as the sheet feeding tray  31  is demounted or mounted. Also, the laser printer  1  may have the pressing plate position sensor  32 A and a sheet remaining amount sensor  32 B (refer to  FIG. 1 ) configured to detect a position of the sheet P, which is located at the uppermost position, of the sheets P placed on the pressing plate  32 , and may be configured to switch the first output control and the second output control on the basis of information from the two sensors. 
     In the above illustrative embodiment, the control operation of the control unit  10  is executed by the CPU. However, the configuration of the control unit is not limited thereto, and a part thereof may be executed by a logical circuit (digital circuit) such as an FPGA (Field Programmable Gate Array), an ASIC (Application Specific Integrated Circuit), a PGA (Programmable Gain Amplifier) and the like.