A weight measuring apparatus can ensure safety even if a trouble or deterioration of measurement accuracy occurs.The weight measuring apparatus (10) measures a weight of fluids for blood purification and includes the followings. A first arm (110) has a first fitting (111) to fit a supply fluid container (20). A second arm (120) is connected to the first arm (110) and has a second fitting (121) to fit a filtrate container (30). A third arm (130) is connected to the second arm (120). A first measuring device (220) measures, as a first total weight, a total weight of the supply fluid container (20) and the filtrate container (30) based on a change of the second arm (120). A second measuring device (230) measures, as a second total weight, the total weight based on a change of the third arm (130). An alarm (300) alarms, when the first and second total weights have different values having a difference equal to or greater than a predetermined value.
