An electronic circuit includes an input terminal, a transimpedance amplifier connected to the input terminal and including an amplifier and a feedback resistor, a first time constant circuit smoothing an output from the transimpedance amplifier, a gain control circuit arranged between the input terminal and ground potential and controlling current flowing between the input terminal and the ground potential on the basis of the output from the first time constant circuit, and a safeguard circuit controlling the gain control circuit and blocking the current flowing between the input terminal and the ground potential, when a signal to be input into the input terminal is stopped.