To provide a low-cost synchronous type magnetic joint for a speed reducer, small-sized, lightweight, and obtaining large transmission torque.

In this synchronous type magnetic joint, permanent magnets are disposed opposite to each other, and both magnets are magnetically connected to transmit torque. The joint includes: a permanent magnet rotor fixed to a driven shaft having a small gear and a drive side permanent magnet rotor rotatably supported on the driven shaft through a bearing device, wherein in the driven side permanent magnet rotor, the permanent magnet is mounted on a discoidal support in the circumferential direction, and in the drive side permanent magnet rotor, the permanent magnet is mounted on a discoidal support having a large gear in the circumferential direction.