In one embodiment, a transistor driver includes a first line as a power supply line configured to supply a first voltage, a second line as a power supply line configured to supply a second voltage, and a first controlling transistor provided between the first line and the second line and configured to control a gate voltage of a first transistor to be driven. The driver further includes a third line configured to supply a third voltage that controls a gate voltage of the first controlling transistor, and a power supply circuit connected to the first, second and third lines and configured to vary the third voltage. The driver further includes a depletion P-type transistor including a gate connected to the first line, a drain connected to the second line, and a source connected to the third line.