PURPOSE: To cause the quality of permeated water to build up immediately after the start of passing water by making a raw water outflow port directly opposite to a raw water inflow port in the circumferential direction of a case and inclining the inner surface of a cast resin of each end of the case from the surface at the position in contact with the raw water outflow port in the inside direction of the case.

CONSTITUTION: A raw water inflow port 41 at one end of a case 1 and a raw water outflow port 42 at the other end are separated from each other about 180° in the circumferential direction of a case. The raw water inflow port 41 is adjacent to a cast resin 31 on one end side of the case. The inner surface 311 of the injection resin 31 is inclined in a diagonal line or a curved line in the inside direction of the case from a plane (p) at the position in contact with the raw water inflow port 41. The raw water outflow port 42 is adjacent to the cast resin 32 on the other end side of the ='; The inner surface 321 of the cast resin 32 is inclined in a diagonal line or a curved line in the inside direction of the case from a plane (p') at the position in contact with the raw water outflow port 42. Therefore, raw water flows without delay in the case and seal water and air before the start of passing water are rapidly replaced by the passed raw water to cause the quality of permeated water to build up in a short time.