A data decoding method judges a signal state where there is a transition from a low level to a high level or from a high level to a low level at the center portion of a bit interval as logical “1” or “0”, and a signal state where a low level continues or a high level continues over the entire bit interval as logical “0” or “1”. The method has the steps of: measuring a first time duration in which the bit series signal transitions from a low level to the next low level, measuring a second time duration in which the bit series signal transitions from a high level to the next high level, and deciding a logical “0” or “1” value for the target bit to be decided based on the combination of the first time duration and the second time duration measured for the target bit.