A sense amplifier of this invention has a main characteristic feature in that it has low power consumption in an input waiting state, and can perform a highspeed sensing operation. The sense amplifier includes an output transistor, a constant current source connected between the base of the transistor and a first power source, a MOS transistor, having a source-drain path connected between the base of the transistor and a second power source, for receiving most of a current from the constant current source, and a load resistor for the transistor. The base potential of the output transistor in the input waiting state is set to be a value corresponding to a state immediately before or after the transistor is turned on.