A pattern selector valve (32) is lodged in a seat supporting stool (22) which supports an operator's seat (23) thereon, with a switch lever (32B) at a front side of the seat supporting stool. An operator can easily check out the position of the switch lever (32B) of the pattern selector valve (32) at the time of getting onto the operator's seat (23). Besides, the switch lever (32B) of the pattern selector valve (32) can be easily changed over to a different pattern position in the vicinity of the operator's seat (23). Check-out and change-over of the switch lever (32B) are facilitated and can be carried out in a secure manner under guidance of a pattern indicator (39) which is provided on a front cover (34).