To minimize the differential pressure between both side surfaces of an impeller housed between an outer case and an inner case in a regenerative pump.

A suction port 9e is formed in the outer case 9, and a C-shaped first passage 9d opposed to an impeller blade part 10a and having one end communicating with the suction port 93, a recessed part 9b located on the inside of the outer-side first passage 9d, and a radially long second passage 9h allowing the recessed part 9b to communicate with the other end of the outer-side first passage 9e are formed on the inside surface of the outer case 9. A delivery port 7e us formed on the inside diameter side of the inner case 7, and a C-shaped first passage 7g opposed to the impeller blade part 10a and a radially long second passage 7h allowing the other end part of the inner-side first passage 7g to communicate with the delivery port 7e are formed on the inside surface of the inner case 7. The second passages 9h and 7h are constituted so as to have mutually different passage sectional areas.