To accurately detects the amplitude value of AC signal waveform having less variations, related to a method and device for detecting wave-height value of an AC signal, and a power source and heating device for an image forming device.

A comparison part 3 comprises a sine wave signal, outputted from a transformer T1 with output value of a slice level output part 2, for generating a binarized pulse signal of H or L level. A microcomputer 10 comprises a sampling part 5 which samples the pulse signal at a prescribed timing, a pulse width measuring part 6 which measures the pulse width of the sampled pulse signal, a measurement value storing part 7 which stores/ accumulates the data of pulse width measured by the pulse width measuring part 6, an averaging part 8 which averages the data stored and accumulated a plurality of number times by the measurement value storing part 7, and a slice level changing part 9 which changes slice level, based on the value of averaged data for the pulse width outputted from the averaging part 8.