A grass mowing machine includes a mower (4) disposed under a machine body and configured to discharge cut grass pieces sent from a discharging section (34) to the outside, a duct (35) connected to the discharging section (34) to be in communication thereto and configured to guide the cut grass pieces from the discharging section (34) and a grass collecting section disposed at a rear portion of the machine body and configured to collect the cut grass pieces guided by the duct (35). A deformation preventing member (H) is provided for preventing posture deformation at a connecting portion of the duct (35) for its connection to the discharging section (34).