In a gate driver for driving a first transistor with a constant current, a constant current circuit supplies the constant current to a gate of the first transistor. A second transistor has a gate supplied with a gate reference voltage as a reference for a drive voltage to turn ON the first transistor and is connected in a forward direction in a path from the constant current circuit to the gate of the first transistor. A controller drives the first transistor by operating the constant current circuit when an ON instruction is inputted. The controller sets the gate reference voltage to a first value when the ON instruction is inputted and then changes the gate reference voltage to a second value greater than the first value when a predetermined timing comes under a condition where the first transistor is not in an overcurrent state.