Abstract: A photodetector amplifier circuit that converts a photodetector signal photoelectrically into a photocurrent and controls on or off state of an output transistor according to the photocurrent. The photodetector amplifier circuit has a first control circuit that is connected to a control terminal of the output transistor and controls on or off state of the output transistor according to the photocurrent and a switch that is connected between the control terminal of the output transistor and a ground voltage terminal and if the output transistor is turned off, discharges the control terminal of the output transistor by making the control terminal of the output transistor conductive with the ground voltage terminal.