An amplifying device comprises a differential amplifier stage, which includes a first transistor supplied with an input signal and a second transistor supplied with an output signal as a negative feedback signal, and a third transistor for detecting cutoff of a power supply voltage to be supplied to the differential amplifier stage. The third transistor controls a fourth transistor which sets the potential at the base of the first transistor at the ground level. The base of the first transistor is connected to the base of a fifth transistor, which controls a sixth transistor connected between the ground and the base of the second transistor so that in a power-off state, the fourth and sixth transistors render the potentials at the bases of the first and second transistors to be the ground level.