To provide a core material of a rotary machine which uses a magnetic steel plate of a high yield as a material for processing and which can be processed with a little processing force and which requires a relatively few assembly processes for a core and which realizes a good performance of a magnetic circuit and also provide a method for manufacturing a core.

Teeth 4 is formed at specified intervals on one side of a belt-like magnetic steel plate. In part of the magnetic steel plate which are located between each two adjacent teeth 4, tapered cutouts 6 are formed from one side where the teeth are formed toward the other side. Where the number of the teeth 4 is α, an opening angle 73 of each cutout 6 is set to 360/α. Because of the existence of the cutouts 6, only a little force is required for processing. Although there are the cutouts 6, a good magnetic circuit is formed since two sides 6a, 6b which constitute the cutout 6 are brought into contact with each other when the plate is actually used.