Disclosed is an autonomous lawn mower (10) in which a movable cover (13) covering a traveling frame (11) is held by a plurality of holding devices (20A, 20B, 20C) so as to be displaceable in the horizontal direction and vertical direction. The plurality of holding devices each include a holding shaft (30) and a shaft holding unit (50). The holding shaft vertically extends, and includes a lower half portion (32) and an upper half portion (33). The shaft holding unit holds the lower half portion accommodated in an accommodating portion (40) of the traveling frame such that the lower half portion can slide in only the horizontal direction. The upper half portion extends through a through hole (13a) 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 compression coil spring (81) intervenes between a spring receiving portion (82) formed in the upper end of the upper half portion and an upper wall (72) of the movable cover. One holding device includes a shaft displacement amount detector (90) which detects the slide displacement amount of the lower half portion.
