An autonomous lawn mower includes a movable cover covering a traveling frame and holding devices holding the movable cover. Each holding device includes a holding shaft and a shaft holding unit. The holding shaft includes a lower half portion and an upper half portion. The shaft holding unit holds the lower half portion accommodated in an accommodating portion of the traveling frame such that the lower half portion can slide in only a horizontal direction. The upper half portion extends through a through hole of the movable cover by extending upward from the traveling frame, thereby holding the movable cover so that the movable cover is vertically displaceable. A coil spring intervenes between a receiving portion and an upper wall of the movable cover. One holding device includes a detector which detects the slide displacement amount of the lower half portion.
