There is provided a signal processing device. Data indicative of a past sensing point, and a counter value indicative of existence possibility of the past sensing point are stored in a storage. Whether the past sensing point has continuity to a recent sensing point is determined. The counter value associated with the past sensing point determined as having no continuity to the recent sensing point is decreased. The data is deleted from the storage means when the counter value becomes less than a first threshold value. The sensing point possibly detected by pairing peak signals obtained in first and second periods in a wrong manner is identified as a specific sensing point. A first value is decreased from the counter value associated with the past sensing point, and a second value different from the first value is decreased from the counter value associated with the specific sensing point.