To compactly constitute a pressure accumulating device for accumulating fluid of converting pressure by a pressure converting mechanism in a pressure accumulator.

The pressure converting mechanism 20 sucks the atmosphere in a pressure converting chamber PR via a flow passage 41e of a cylinder 41 in a selector valve mechanism 40, first and second piston flow passages 42b and 42c and a suction valve 46 by reciprocating motion of a piston 22, and supplies air converted into high pressure by a pressure converter PR to a first chamber R1 and an accumulator 12 of a cylinder 41 via the first and second piston flow passages 42b and 42c and a delivery valve 47. When air pressure in the accumulator 12 and the first chamber R1 becomes predetermined pressure or more, a piston 42 is displaced rightward (illustrated) against a coil spring 43. Since the pressure converting chamber PR communicates with the atmosphere via flow passages 41f and 41e of the cylinder 41 by rightward displacement of the piston 42, the pressure converting mechanism 20 stops pressure conversion.