An apparatus has an input hold circuit for sequentially holding digital feedback signals sequentially transmitted at a sampling period, and for sequentially outputting the transmitted feedback signals or held feedback signals at a period which is shorter than the sampling period; a notch filter for applying a filtering processing to the feedback signals outputted from the input hold circuit; and an output hold circuit for sequentially outputting the feedback signals subjected to the filtering processing by said notch filter, while thinning the same, at a period which is same as the sampling period.