A threshold value at a side of a bottom value of a sensor signal is renewed by a value calculated by multiplying a minimum value of a deviation between the bottom value and a reference value for the predetermined period of time by a ratio when the minimum value of the deviation is equal to or smaller than the threshold value. A threshold value at a side of a peak value thereof is also renewed by a value calculated by multiplying a minimum vale of a deviation between the peak value and the reference value from the predetermined period of time by a ratio when the minimum value of the deviation is equal to or smaller than the threshold value.