An electromagnetic reciprocating fluid apparatus includes a magnetic circuit device in which magnetic pole members project inward from an annular magnetic circuit member so as to oppose each other. The magnetic pole members and the annular magnetic circuit member have a uniform thickness. First and second additional annular magnetic circuit members are set on both sides of the annular magnetic circuit member to constitute a part of the magnetic circuit device, thereby minimizing the increase in magnetic reluctance due to magnetic flux concentration at the joints between the magnetic pole members and the annular magnetic circuit member.