A surgery assistance device (1,2) comprising: an operation input section configured to output a manipulation order based on input from a user; a motion section (30, 143) which is formed with a channel (50, 146) into which a surgical instrument is insertable, the surgical instrument being capable of moving through the channel (50, 146), the surgical instrument provided with a treatment section at a distal end portion of a flexible surgical instrument insertion section; a movement amount detection means detecting a treatment section movement amount which is at least one of a protrusion length by which the treatment section of a surgical instrument inserted into the channel (50, 146) protrudes forward from a distal end opening (68a, 145a) of the channel (50, 146) and a rotation angle of the surgical instrument about an axis of the channel (50, 146); a motion section information calculation means calculating motion section information which is capable of specifying a position and an orientation of a distal end of the motion section (30, 143); and a motion control unit controlling motion in the motion section (30, 143) based on the manipulation order, wherein the operation input section comprises: a detection target object attachable to an operation section, the operation section operating the treatment section and being fixed to a proximal end portion of the surgical instrument insertion section; and a detection section (12, 155) detecting the detection target object, wherein the detection section (12, 155) calculates detection target object information capable of specifying a position and orientation of the detection target object, the operation input section outputs the manipulation order based on the detection target object information calculated by the detection section (12, 155), and characterized in that the motion control unit controls motion in the motion section (30, 143) based on the detection target object information of the manipulation order, the treatment section movement amount detected by the movement amount detection means, and the motion section information. an operation input section configured to output a manipulation order based on input from a user; a motion section (30, 143) which is formed with a channel (50, 146) into which a surgical instrument is insertable, the surgical instrument being capable of moving through the channel (50, 146), the surgical instrument provided with a treatment section at a distal end portion of a flexible surgical instrument insertion section; a movement amount detection means detecting a treatment section movement amount which is at least one of a protrusion length by which the treatment section of a surgical instrument inserted into the channel (50, 146) protrudes forward from a distal end opening (68a, 145a) of the channel (50, 146) and a rotation angle of the surgical instrument about an axis of the channel (50, 146); a motion section information calculation means calculating motion section information which is capable of specifying a position and an orientation of a distal end of the motion section (30, 143); and a motion control unit controlling motion in the motion section (30, 143) based on the manipulation order, wherein the operation input section comprises: a detection target object attachable to an operation section, the operation section operating the treatment section and being fixed to a proximal end portion of the surgical instrument insertion section; and a detection section (12, 155) detecting the detection target object, wherein the detection section (12, 155) calculates detection target object information capable of specifying a position and orientation of the detection target object, the operation input section outputs the manipulation order based on the detection target object information calculated by the detection section (12, 155), and characterized in that the motion control unit controls motion in the motion section (30, 143) based on the detection target object information of the manipulation order, the treatment section movement amount detected by the movement amount detection means, and the motion section information. a detection target object attachable to an operation section, the operation section operating the treatment section and being fixed to a proximal end portion of the surgical instrument insertion section; and a detection section (12, 155) detecting the detection target object, wherein the detection section (12, 155) calculates detection target object information capable of specifying a position and orientation of the detection target object, the operation input section outputs the manipulation order based on the detection target object information calculated by the detection section (12, 155), and characterized in that the motion control unit controls motion in the motion section (30, 143) based on the detection target object information of the manipulation order, the treatment section movement amount detected by the movement amount detection means, and the motion section information.