A coupling member comprises a spherical rotatable valve member installed
in a fluid passage thereof, in which the fluid passage is maintained open
when a through hole of the valve member is aligned with the fluid
passage, while the fluid passage is maintained closed when the through
hole of the valve member not aligned with the fluid passage. An annular
sealing convex portion is arranged on the outer surface of the valve
member, which has larger radii relative to a pivotal axis A than the
outer surface, so that the sealing convex portion is fully engaged with
an end-opening peripheral edge of the fluid passage only when the valve
member is placed in the closing rotational position.