in fig1 designate same or like parts and components in fig2 a 3b reference numeral 31 designates a holder for use in the embodiment of the invention and 32 a valve for the same the holder 31 has a substantially cylindrical form as the conventional example and includes on an inner peripheral portion a female screw to be screwed with a male screw of a rotor shaft 3 the holder 31 is supported by the rotor shaft 3 while inserted thereto at the front end of the holder 31 an outer sleeve 33 and an inner sleeve 34 for holding the valve 32 are formed around an opening edge at the front of the outer sleeve 33 is a collar 33a arranged to prevent the valve from being dropped in this embodiment the collar 33a has a notch now shown so that a valve stopper described later can be fitted therewith the valve 32 has a cylindrical guide 35 at its rear end the guide being inserted into the gap formed between the outer sleeve 33 and inner sleeve 34 of the holder 31 on an outer peripheral portion of the valve 32 a stopper 36 is formed to be fitted with the collar 33a of the outer sleeve 33 once the guide 35 has been inserted into the gap between the outer sleeve 33 and the inner sleeve 34 and the stopper 36 has been fitted with the collar 33a the valve 32 can be supported by the holder 31 so as.