PURPOSE: To improve a space factor of a core by circumferentially laminating metal magnetic steel plates bent in an arc state and molded, and so forming it that its lateral section becomes a ring state.

CONSTITUTION: A core is formed by laminating core pieces 1 formed by bending metal magnetic thin plates in an arc state and molding and so forming it that its lateral section becomes a ring state. In this case, a circumferential length component per one piece 1 is its thickness itself at an inner periphery r=r1, increased at a position of a radius (r) to (thickness × secθ) so that, when the radius (r) is increased, secθ becomes large and even if a thickness is constant, the difference of a perimeter is corrected by providing an angle. Thus, since a ringlike core having no space and large space factor can be formed, an electromagnetic pump can be reduced in size and enhanced in performance.