A gear transmission (20) includes a plurality of shift gears (36, 37), a plurality of shifters (55, 56) engaged, respectively, with the plurality of shift gears (36, 37), and a shift drum (58) rotatably operated by a stepped speed changing operational tool (66) to operate the plurality of shifters (55, 56), thus being speed-changed in forward three speed stages. The stepped speed changing operational tool (66) is switched to a neutral position, a forward third speed position, a forward second speed position and a forward first speed position in this cited order.