PURPOSE: To prevent a multiple disk clutch from burning in a driving force transmission by avoiding a locking-up condition during usual driving at a low temperature, and preventing locking-up even when a differential rotation speed which forms a locking-up condition at a high temperature.

CONSTITUTION: A cam engagement element 29 which is engaged with the cam surface 21A in a circumferential direction which is provided at the edge wall of a housing 21 is installed in the housing 21 so as to generate thrust for pressing a multiple disk clutch 25 by relative rotation with the housing 21. A number of outer plates 37 of the multiple disk clutch 25 are engaged with the cam engagement bodies 29 in a rotation direction respectively, and plural inner plates 38 of the multiple disk clutch which is disposed between respective outer plates 37 are engaged with rotation shafts 22 respectively. The driving force transmission is provided with a spring 30 which is constituted of bimetal for energizing the cam engagement bodies 29 by spring load in response to a temperature in a direction where the housing 21 is engaged with the cam surface 21A.