To supply AC power to a load apparatus without interruption, even when a microcomputer, as a control subject of an inverter circuit, is in a functional incompetence.

An uninterruptible power supply unit includes power receiving terminals 11, 12, power storage means, power supplying terminals 13, 14, physical quantity detecting means, triangular wave signal generating means, sinusoidal signal generating means, operation amount specifying means, comparing means for specifying a first time having a smaller amplitude of the triangular wave signal and a second time having a greater amplitude of the triangular wave signal, a first and fourth switching elements 25, 26, 27, 28 to be switched from off to on when the first time starts, and a second and third switching elements 25, 26, 27, 28 to be switched from off to on when the second time starts.