Provided is a device for performing speed change not leading to engine stall in a working vehicle such as a wheel loader while sustaining a required tractive force and reducing fuel consumption. When a speed (“forward”, “second speed”) suitable for a work is selected by a speed change operator, control means makes a modulation clutch engaged in response to the vehicle speed drop and makes a lock-up state where a lock-up clutch is engaged transit to a lock-up and modulation clutch slip state where the lock-up clutch is engaged while the modulation clutch slips. Subsequently, the control means performs a clutch control for causing a transition to both clutch slip state where the lock-up clutch and the modulation clutch slip in response to the vehicle speed drop, hence causing the transition to a torque converter operation state where the modulation clutch is engaged and the lock-up clutch is released.