To provide a power transmission controller which can suitably control transmission torque capacity between a driving source and a driven member without incurring enlargement or complication.

The power transmission controller, which controls a transmission torque between a driving source 13 and driven members 1, 2, comprises: a planetary roller mechanism 14; a magnetic fluid 26 interposed in a clearance 25 arranged between at least one of a first rotator 21 and a second rotator 22 of its planetary roller mechanism 14 and a rolling element 24; and a transmission torque control means for controlling a transmission torque capacity of the planetary roller mechanism 14 by changing the fluidity of the magnetic fluid 26 by controlling magnetism applied to the magnetic fluid 26. Furthermore, the driving source 13 is connected to transmit power to at least one of the rollers of the planetary roller mechanism 14, and the driven members 1, 2 are connected to power transmission to at least the other trollers to which the driving source 13 of the planetary roller mechanism 14 is not connected.