A mowed grass amount detecting apparatus for a grass collection container of a grass mowing includes a pivotal sensor plate (62) constituting a portion of a boundary wall for holding the mowed grass charged into the grass collection container. The pivotal sensor plate has a receiving face which receives the weight of mowed grass. The apparatus also includes a detecting unit (66) for detecting an amount of pivotal movement of the sensor plate (62) from a home position. The receiving face of the pivotal sensor plate (62) is configured so as to close a sensor opening portion formed downwardly of a mowed grass charging portion of the grass collection container communicated and connected with the conveying duct, so that the receiving face of the pivotal sensor plate receives the weight of mowed grass held within the grass collection container (32) through the sensor opening portion.