the rolling direction u201c a u201d of the ball 41 and the tangential direction u201c 1 u201d of the retaining cavity 42 coincide that is revolution phases of the spiraling scroll 9 and that of the ball 41 are matched the ball begins a revolving motion as shown in fig4 a as above mentioned the ball 41 collides with the interior surface of the retaining cavity 42 but the noise during the startup is reduced because the ball retaining device 40 is made of polymeric resin here in the above embodiment although the diameter of the retaining cavity 42 provided in the retaining device 40 is given by d d u03c1 it is only necessary to select the diameter d so as not to interfere with the rolling motion of the ball 41 and therefore it is sufficient for the relation to be d u2267 d u03c1 when d d u03c1 ball 41 rolls while touching the interior surface of the retaining cavity 42 and when d u2267 d u03c1 ball 41 rolls without touching the interior surface of the retaining cavity 42 further the interior surface of the retaining cavity 42 may be shaped in a shape other than a cylindrical shape as described below that is as shown in fig5 the interior surface of the retaining cavity 42.