PURPOSE: To allow a retarder and an exciter to be fixed at optimal positions by spacing them apart from a shaft by a predetermined distance.

CONSTITUTION: A parking brake 13 can be fixed to a shaft 9 through the use of a space 12 formed between the main parts of the shaft 9 and a retarder 7 and the main part of an exciter 19. In other words, the parking brake 13 and the retarder 7 equipped with the exciter 19 can be installed for the shaft 9 in the rear of a transmission 8. Consequently, the parking brake 13 and the retarder 7 can be juxtaposed without modifying the fixing position of the parking brake 13 and the retarder 7 can be fixed at an optimal position, i.e., in the rear of the transmission 8. This constitution enhances deceleration performance of vehicle, especially of truck.