A weight sensor of this invention can balance among weights of plural substances and calculate the respective weights. The weight sensor includes: an arm (23) having an end fixed to a pillar (22) and a free end； a filtrate holder (24), a replacement fluid holder (25), and a dialysate holder (26) provided at three locations of the arm (23) along a longitudinal direction of the arm to hold respective substances； a first strain value sensor (27), a second strain value sensor (28), and a third strain value sensor (29) each of which detects a strain value of the arm (23) corresponding to a total weight of the substances held by the holders ranging from a holder proximate to the free end to a corresponding holder； and a weight calculation unit (30) that calculates the total and each of the weights from the obtained detection results.
