To provide a magnetic particle device reducing a whole weight thereof while maintaining an operation surface area, and service life.

This magnetic particle device 10 transmits a torque between two rotational members 30, 40. This device comprises a fixed housing 12 having a penetrating duct 14 for receiving a rotating shaft 16; and relatively rotational members 30, 40 forming a gap 66 including a magnetic reactive medium 64; and a magnetic source for generating a magnetic flux 68. The rotational members 30, 40 have a plurality of annular grooves 38, 56 forming a plurality of operation surfaces 58, 59, 60, 61. The magnetic flux 68 penetrates the operation surfaces 58, 59, 60, 61 to pass through the gap 66, and to magnetize the magnetic reactive medium 64, so that the rotational members 30, 40 are magnetically connected, thereby realizing transmission of the torque between the rotational members. By providing a number of operation surfaces, largeness and weight of the magnetic source are reduced to realize a more compacted and lightened device.