PURPOSE: To achieve simplification of a lock-mechanism bonnet-side face portion and machine-body side reception portion in shape and reduction in machining cost of these portions by constructing the lock mechanism by a first resinous engagement portion of the bonnet-side face lower portion and a second one of the machine-side reception portion.

CONSTITUTION: A lock mechanism A has a structure wherein, in a closed posture of a bonnet 6, it is engaged with and fitted to this bonnet 6 in such a manner as to regulate relative movement between the bonnet 6 and a machine body; and its state of engagement and fitting is released as the bonnet 6 is opened. Specifically, a resinous first engagement portion 8 in the form of a hook whose section is downwardly directed is bonded to a lower portion of a side face 6b of the bonnet 6 while a resinous second engagement member 9 in the form of a projection whose section is shaped like a laterally directed circular arc is bonded to side faces 7a of right/left rear fenders. Since the lock mechanism A has such a structure, its right left movement due to upward shifts of both the engagement members 8, 9 resulting from opening/closing of the bonnet 6 is absorbed by displacement of the side face 6b in the direction of a plane thereof. Accordingly, the lock mechanism A becomes simple in structure, and therefore become light in weight and low in cost.