A grass mower comprising: a drive wheel unit; a variable speed drive mechanism (21, 22) for driving the drive wheel unit; a mower unit having a cutter blade; a cuter blade drive mechanism for rotatably driving the mower unit; a man-operable vehicle speed controlling tool which is operated in a range from a zero vehicle speed position to a maximal vehicle speed position; an operation detection section for detecting an operational position of the man-operable vehicle speed controlling tool; a vehicle speed instruction value calculation section for generating a vehicle speed instruction value including a maximal vehicle speed, based on the operational position of the man-operable vehicle speed controlling tool detected by the operation detection section; a mode selection section for selecting a control mode from a plurality of control modes; a control table in which a plurality of control parameters are set, the control parameters being provided for the respective control modes, the respective control parameters including a rotational speed value of the cutter blade and a maximal vehicle speed value corresponding to the rotational speed value; a vehicle speed control target value calculation section configured to calculate a vehicle speed control target value, based on a vehicle speed instruction value generated by the vehicle speed instruction value calculation section and a selected control parameter corresponding to the control mode selected by the mode selection section; a traveling control section configured to control the variable speed drive mechanism (21, 22) based on the vehicle speed control target value; a cutter blade control target value calculation section configured to calculate a rotational speed control target value for the cutter blade, based on the selected control parameter; a cutter blade control section configured to control the cutter blade drive mechanism based on the rotational speed control target value; and a cutter blade speed detection section configured to detect and output a rotational speed of the cutter blade; characterized by a maximal vehicle speed value changing section for changing the maximal vehicle speed value in the control table, based on a cutter blade rotational speed of the cutter blade detected by the cutter blade speed detection section, and the vehicle speed control target value calculation section being configured to, if the man-operable vehicle speed controlling tool has been operated to the maximal vehicle speed position, output as the vehicle speed control target value the maximal vehicle speed value as changed by the maximal vehicle speed value changing section. a drive wheel unit; a variable speed drive mechanism (21, 22) for driving the drive wheel unit; a mower unit having a cutter blade; a cuter blade drive mechanism for rotatably driving the mower unit; a man-operable vehicle speed controlling tool which is operated in a range from a zero vehicle speed position to a maximal vehicle speed position; an operation detection section for detecting an operational position of the man-operable vehicle speed controlling tool; a vehicle speed instruction value calculation section for generating a vehicle speed instruction value including a maximal vehicle speed, based on the operational position of the man-operable vehicle speed controlling tool detected by the operation detection section; a mode selection section for selecting a control mode from a plurality of control modes; a control table in which a plurality of control parameters are set, the control parameters being provided for the respective control modes, the respective control parameters including a rotational speed value of the cutter blade and a maximal vehicle speed value corresponding to the rotational speed value; a vehicle speed control target value calculation section configured to calculate a vehicle speed control target value, based on a vehicle speed instruction value generated by the vehicle speed instruction value calculation section and a selected control parameter corresponding to the control mode selected by the mode selection section; a traveling control section configured to control the variable speed drive mechanism (21, 22) based on the vehicle speed control target value; a cutter blade control target value calculation section configured to calculate a rotational speed control target value for the cutter blade, based on the selected control parameter; a cutter blade control section configured to control the cutter blade drive mechanism based on the rotational speed control target value; and a cutter blade speed detection section configured to detect and output a rotational speed of the cutter blade; characterized by a maximal vehicle speed value changing section for changing the maximal vehicle speed value in the control table, based on a cutter blade rotational speed of the cutter blade detected by the cutter blade speed detection section, and the vehicle speed control target value calculation section being configured to, if the man-operable vehicle speed controlling tool has been operated to the maximal vehicle speed position, output as the vehicle speed control target value the maximal vehicle speed value as changed by the maximal vehicle speed value changing section.