A monitoring circuit of the present invention provides a monitor signal with which a magnitude of a current flowing across a photodetector, such as a photodiode, can be calculated accurately over a wide temperature range on the basis of a value of the monitor signal. The monitoring circuit of the present invention includes: a current mirror circuit for outputting a monitor current proportional to an input current, the current mirror circuit having an input point for receiving the input current, the input point being connected to a photodetector and a load resistor, which are connected thereto in parallel; and an output circuit for outputting a monitor signal indicating a difference between a monitor electric potential proportional to the monitor current, and an offset electric potential proportional to an offset current which flows across the load resistor concurrently with the monitor current.