To provide an AC uninterruptive power circuit, normally supplied with a commercial power which can normally switch the supply power, when an input power supply is in an off state and which can prevent unnecessary switching of the power supply when noise is superposed on the input power supply.

This power circuit is provided with an input zero-crossing detecting section 6a as a means for detecting an abnormality of the imput power supply, an input voltage judging section 6b while judges whether the input voltage is normal or abnormal, a measuring and string section 6c which measures and stores an input voltage value of each sampling cycle, an abnormal change detecting section 6c which detects an abnormal change in the input voltage in sampling, and a counter 6d which counts abnormalities of the input voltage at the sampling cycles, based on the abnormal change in the input voltage and outputs an abnormal detection signal, when the number of abnormalities reaches a specified number of times.