A pump with high performance and cleanability includes a casing having a smaller volute and a larger volute; a space between an outer circumference of an impeller and a starting end of the smaller volute being greater than that of the larger volute, generating a circulating flow of self-priming water from the smaller volute to the larger volute; and a diffusing part of the larger volute being formed into an upright, cylindrical self-priming water separating chamber guiding the self-priming water from the smaller volute to flow in for air-water separation. An inner circumference part of the casing is formed concentric with the outer circumference of the impeller with a predetermined space therebetween; defining members are protrusively disposed on the inner circumference part of the casing so as to define the shapes of the two volutes; and the self-priming water separating chamber is made attachable to and detachable from the casing.