A noise filter, used for a digital signal, configuring an [N-1]-stage filter by connecting the N-number of flip flops in series comprises a selection unit for outputting a filter output signal by selecting one of signals filtered by one to (N-1) stages based on a selection signals; and a wave form check unit for making the selection unit change a selection thereby by changing a value of the selection signal based on a pulse width on the filter output signal which is output from the selection unit.