To reduce cogging torque effectively in a motor structure in which field bodies are provided inside and outside of an annular core, and the number of slots is larger than the number of magnetic poles.

A brushless motor 1 includes: a stator 8 comprising teeth Ti and To arranged on the outer periphery and inner periphery of an annular core 2; a shaft 4; a first field body 13 placed between the shaft 4 and stator 8; and a second field body 14 placed outside of the stator 8. The first field body 13 and second field body 14 comprise a plurality of magnet pieces, and the number of slots is larger than the number of magnetic poles. By adjusting the opening angle of each magnet piece, it is operated such that a first cogging torque waveform indicating the change trend of cogging torque generated between the first field body 13 and stator 8 relative to the rotational angle of the shaft 4 and a second cogging torque waveform indicating the change trend of cogging torque generated between the second field body 14 and stator 8 will be mutually reversed in the order of positive and negative of the peak torque values after the shaft 4 started to rotate from a reference position.