The purpose of the invention is to provide a self-propelled grass mower that can perform precise guidance control by reliably detecting a guidance signal with a guidewire. Provided is a self-propelledgrass mower that comprises a wheel motor to drive a wheel, a cutting blade motor to drive a cutting blade, a rechargeable battery to supply electrical power to these motors, and a guidewire sensor todetect a magnetic field generated by a guidewire formed into a loop, and that detects whether the device is inside or outside of a region enclosed by the guidewire and runs autonomously in a grass cutting region, wherein the supply voltage is reduced only for the cutting blade motor when a magnetic field is detected by the guidewire sensor. Driving of the cutting blade motor is restarted after detection of the magnetic field has completed. Thereby, the impact of noise due to the cutting blade motor is eliminated.