A body motion determination system for determining whether a body motion of a subject (S) on a bed (BD) presents or not, includes a load detector (11, 12, 13, 14) configured to detect a load of the subject on the bed； a non-negative-valued average calculating unit (313) configured to calculate a non-negative-valued average of a detection value of the load detector； and a threshold value setting unit (315) configured to set a threshold value to be used in determining whether the body motion of the subject presents or not, based on the non-negative-valued average calculated in a resting period in which the subject merely performs a respiration.
