A transmission includes an input shaft, an output shaft and an intermediate shaft, a power transmission mechanism and a control unit. The power transmission mechanism includes a forward travelling lower speed clutch, a forward travelling higher speed clutch, a rearward travelling clutch and a plurality of speed stage switching clutches. The control unit is configured to switch a gear state into a shiftable rearward travelling gear stage by turning off either the forward travelling lower speed clutch or the forward travelling higher speed clutch and by turning on the rearward travelling clutch in executing an operation of switching from forward travelling to rearward travelling, and to switch a gear stage into a shiftable forward travelling gear stage by turning off the rearward travelling clutch and turning on the forward travelling lower speed clutch in executing an operation of switching from rearward travelling to forward travelling.