25 in the molt pressing body 23 and flows through the moltpren 22 a capsule accommodating cylinder 26 is provided integrally under the moltpren pressing body 23 and the capsule accommodating cylinder 26 contains a capsule 2 which holds the membrane film 1 shown in fig2 although in the indicated embodiment the capsule shown in fig2 is accommodated needless to say it is permissible to use the capsule shown in fig1 a gasket 27 seals between the accommodated capsule 2 and the moltpren pressing body 23 a throttling member 48 is inserted into the capsule 2 reference numeral 28 denotes a flow rate adjusting body which is inserted into a fixed cylinder 10 movably in the vertical direction while its bottom end is projected outwardly through a bottom wall of the storage tank 11 and which can be rotated from outside a top end of the flow rate adjusting body 28 comes in contact with a bottom face of the throttling member 48 incorporated in the capsule 2 through a gasket 29 a vertical motion of the flow adjusting body 28 is transmitted to the molt pressing body 23 through the gasket 29 the throttling member 48 and the capsule 2 and capable of pressing the moltpren 22 a through central passage 30 is made in the central portion of the flow rate adjusting body 28 and communicates with the inside of the tank 11 through a lateral hole 31 an elastic porous body.