into the tank 1 even if the pump is left in an inoperative position for a long time the disc 25 may be eliminated by modifying the plunger to the structure shown in fig1 a in which a disc 24 39 is accompanied with a seal member such as an oil seal s which is excellent in sealing function and enhances the sealing effect thereof after all the sample has been put into the tank 1 pressurized air is supplied to the tank for a predetermined time agitation by pressurized air insures that the components in the sample stock are evenly dispersed in the water in the tank 1 including the cylindrical portion 6 and a uniform dispersion is obtained in a short time thereafter the valve v1 communicating with the case 14 is opened and thereupon the interior of the apparatus is isolated from the atmosphere by closing the valve v6 at the same time slight pressure is caused by opening a valve v7 through the air intake pipe 4 and under such conditions the sample is screened through the screen 13 the liquid which passes through the screen 13 is delivered to the measuring cylinder 17 through an outlet 16 and a conduit 18 after a predetermined time the supply of the pressurized air is stopped and the valve v8 at the inlet of the cylinder 17 is closed.