Abstract: A photosensor circuit comprises a photodiode PD, which converts a light signal into electricity, an n-channel MOS transistor QD1 which has a logarithmic characteristic in its weakly reversed condition and converts a sensor current generated by the photodiode PD into a detection voltage which has a logarithmic characteristic, and a capacitor which is connected to a detection terminal of this transistor QD1. A resetting voltage VG=VS is applied to the gate G of the transistor QD1 to lower the impedance between the drain and the source of the transistor for resetting the transistor to an initial condition. As the n-channel MOS transistor QD1 is a depletion type n-channel MOS transistor, when this transistor QD1 does not receive any gate voltage, the transistor stays in a weakly reversed condition and has a logarithmic characteristic. This photosensor circuit has a high SN ratio and is capable of detecting minute light signals in high precision.