A mower unit provided in a grass mower unit includes a mower deck having a top plate and a side plate extending downwards from the top plate, a rotational shaft extending through the top plate and rotatably supported to the top plate, and a blade attached to the rotational shaft inside the mower deck. A vibration detection unit is attached to the mower deck for detecting vibration in the mower deck. Vibration detected by the vibration detection unit is informed via an informing unit.