To provide a retarder controller which ensures natural decelerating feeling and thus does not give unpleasant feeling to a driver even if a shiftdown operation is suddenly started in some time after a manual-shift-down mode is set.

A control unit 30 for an automatic transmission keeps the level of braking force output by a retarder 13 constant until the automatic transmission 12 starts a shifting-down operation even after a driver sets a manual- selectdown mode by a selector lever 19. When it starts the shifting-down operation, the level of braking force is reduced by an amount Q to prevent decelerating force from increasing excessively. The decrease amount Q is made the larger as the time from the setting of the manual-select-down mode to the start of the shift-down operation is longer, thereby reducing shock caused by the gear shift.