A belt type variable speed-change mechanism transmitting a rotational power from a driving shaft to a driven shaft in a continuously variable manner and a constant-speed power-transmitting mechanism transmitting a rotational power at a constant-speed from the driving shaft to a constant-speed driven-side rotational body supported by the driven shaft in a relatively rotatable manner with respect thereto are accommodated in a casing. A planetary gear mechanism including a first element receiving a variable-speed rotational power through the driven shaft, a second element receiving a constant-speed rotational power through the constant-speed driven-side rotational body and a third element outputting a combined rotational power is supported by the driven shaft in the casing.