An adapter includes a base body including a pair of coupling members and a connecting plug inserted through a hole in the base body and screw-engaged with a port of the two-way valve. In addition, upon threaded engagement of the connecting plug, the base body is mounted with respect to an attachment surface of the two-way valve. Further, a seal member is installed on an end surface of the base body, such that a sealing function is performed by abutment of the seal member against the attachment surface. Moreover, first and second connecting flanges of a connecting apparatus are engaged with respect to the coupling members of the adapter.