A power supply system for an apparatus having a normal mode and a power saving mode comprising: a switching power supply configured to rectify and smooth an alternating current voltage of an alternating current power supply and generate a direct current voltage to the apparatus in the normal mode; a control device; and a small-capacity power supply (30 having a power supply capacity smaller than the power supply capacity of the switching power supply and effective to supply power to the control device; the power supply system being characterized in comprising: a sensor configured to output a detection voltage (Vd) corresponding to a detection target being an environmental temperature and/or humidity (Ta); and the control device comprising: a reference voltage generation circuit (76, 73) configured to generate a reference voltage (Vbs) indicative of a permitted range of the detection target (Ta); a comparison circuit configured to compare the detection voltage (Vd) output from the sensor with the reference voltage (Vbs) and to generate a comparison voltage (Vco) corresponding to the comparison result of the comparison circuit; and a monitoring unit configured to monitor the detection target (Ta) based on the comparison voltage (Vco) during the time period the switching power supply is stopped in the power saving mode. a switching power supply configured to rectify and smooth an alternating current voltage of an alternating current power supply and generate a direct current voltage to the apparatus in the normal mode; a control device; and a small-capacity power supply (30 having a power supply capacity smaller than the power supply capacity of the switching power supply and effective to supply power to the control device; the power supply system being characterized in comprising: a sensor configured to output a detection voltage (Vd) corresponding to a detection target being an environmental temperature and/or humidity (Ta); and the control device comprising: a reference voltage generation circuit (76, 73) configured to generate a reference voltage (Vbs) indicative of a permitted range of the detection target (Ta); a comparison circuit configured to compare the detection voltage (Vd) output from the sensor with the reference voltage (Vbs) and to generate a comparison voltage (Vco) corresponding to the comparison result of the comparison circuit; and a monitoring unit configured to monitor the detection target (Ta) based on the comparison voltage (Vco) during the time period the switching power supply is stopped in the power saving mode. a sensor configured to output a detection voltage (Vd) corresponding to a detection target being an environmental temperature and/or humidity (Ta); and the control device comprising: a reference voltage generation circuit (76, 73) configured to generate a reference voltage (Vbs) indicative of a permitted range of the detection target (Ta); a comparison circuit configured to compare the detection voltage (Vd) output from the sensor with the reference voltage (Vbs) and to generate a comparison voltage (Vco) corresponding to the comparison result of the comparison circuit; and a monitoring unit configured to monitor the detection target (Ta) based on the comparison voltage (Vco) during the time period the switching power supply is stopped in the power saving mode. a reference voltage generation circuit (76, 73) configured to generate a reference voltage (Vbs) indicative of a permitted range of the detection target (Ta); a comparison circuit configured to compare the detection voltage (Vd) output from the sensor with the reference voltage (Vbs) and to generate a comparison voltage (Vco) corresponding to the comparison result of the comparison circuit; and a monitoring unit configured to monitor the detection target (Ta) based on the comparison voltage (Vco) during the time period the switching power supply is stopped in the power saving mode.