The received-light intensity measuring device includes: a pixel circuit 1 including a photodiode (PD) that accumulates an amount of electric charges according to the intensity of received light, a floating diffusion (FD) that generates a signal voltage VCL according to an amount of retained electric charges, and a transfer switch that controls the transfer, to the FD, of the electric charges accumulated in the PD; a DAC 11 that generates a control voltage VTRAN varying in a ramp waveform and applies VTRAN to the gate of the transfer switch; a column AD conversion circuit 13 that obtains the digital value by quantizing a length of time from a first point in time set with reference to a period during which VTRAN is applied to a second point in time at which a specific fluctuation occurs in a temporal variation rate of the signal voltage VCL while VTRAN is being applied.