A speed change system for a working vehicle comprises a main transmission, a sub transmission and a preliminary transmission. The main transmission receives a prime mover power outputted from a prime mover, and has plural main speed stages, one of which is selected to transmit the prime mover power. The sub transmission is disposed on the downstream of the main transmission, and has plural sub speed stages, one of which is selected to transmit a power from the main transmission. The preliminary transmission is disposed on the upstream of the main transmission, and has plural preliminary speed stages, one of which is selected to transmit the prime mover power to the main transmission. An operation for selecting one of the preliminary speed stages can be performed during traveling of the working vehicle.