PURPOSE: To provide a magnetic friction roller type speed reducer allowing positive transmission to be obtained for a long period of time without complicated elastic pressure contact mechanism between rollers so as to be made compact and prevented from the easy wear of the rollers capable of moving to adjust automatically even if wear is generated.

CONSTITUTION: A pair of iron rings 12 are rigidly secured being respectively brought close to the side faces of the N-pole side and S-pole side of a permanent magnet ring 11, and the outer edges of these iron rings 12 are protruded from the outer periphery of the permanent magnet ring 11 to form a conical planetary magnetic friction roller 8. The planetary magnetic friction roller 8 is brought into auto-rotatable and revolvable contact with inclined annular rolling faces 1a, 4a. When the planetary magnetic friction roller 8 or the annular rolling faces 1a, 4a wear off, the planetary magnetic friction roller 8 is moved by its magnetic attracting force relatively in the axial direction to the annular rolling faces 1a, 4a so as to maintain the appropriate contact state to the annular rolling faces 1a, 4a.