A weight sensor includes an arm having an end fixed to a pillar and a free end; a filtrate holder, a replacement fluid holder, and a dialysate holder provided at three locations of the arm along a longitudinal direction of the arm to hold respective substances; a first strain value sensor, a second strain value sensor, and a third strain value sensor each of which detects a strain value of the arm 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 that calculates the total and each of the weights from the obtained detection results.