A laser diode drive includes a first photo diode connection terminal to connect a first photo diode that detects light emission amounts of multiple laser diodes, multiple second photo diode connection terminals to connect multiple second photo diodes that detect light emission amounts of the respective multiple laser diodes, multiple APC controllers to control the light emission amounts of the multiple laser diodes based on monitor currents from the first photo diode or the second photo diodes, multiple switches to connect and disconnect the respective APC controllers with a monitor current path formed between the first photo diode connection terminal and the multiple APC controllers, and a detector to detect whether or not the first photo diode is connected to the first photo diode connection terminal and cause all of the switches to disconnect when the first photo diode is not connected to the first photo diode connection terminal.