An arithmetic processor is provided to detect a background even when observed data values vary. The processor includes a background candidate data memory 20 storing data as a background candidate. In the data, an observation count, as the number of times of obtaining the same observation data, is correlated with distance data representing the observed data. The processor also includes a ranking section 11 ranking background candidates using the observation count as a basis; an observation count update section 12 comparing observed data with a background candidate stored in the memory 20 to find a match, counting observations of the background candidate if there is a match, and replacing distance data of a lowest-ranked background candidate with the observed data if there is no match; and a background determination section 13 determining there are two backgrounds if the top two background candidates remain unchanged for a predetermined period.