In a bandgap reference voltage generating circuit having first, second and third unitary circuits connected in parallel between a power supply voltage and a ground, there is added a fourth unitary circuit including an n-channel FET turned on in response to a bias voltage applied to a gate of the n-channel FET. The second unitary circuit is connected to the fourth unitary circuit through a capacitor having one end connected to a drain of the n-channel FET. When the bias voltage is applied to turn on the n-channel FET of the fourth unitary circuit, since the potential of the one end of the capacitor is dropped, a gate potential of n-channel FETs included in the first and second unitary circuits and operating in a weak inversion condition quickly becomes definite, so that a reference voltage can be generated quickly.