An autonomous lawn mower in which a movable cover configured to cover an upper portion and side portions of a traveling frame provided with traveling wheels is held by a plurality of holding devices such that the movable cover is displaceable in a horizontal direction and a vertical direction with respect to the traveling frame , wherein each of the plurality of holding devices comprises: a holding shaft formed by a single rod-like member having an axis which vertically extends with respect to the traveling frame , and having an upper half portion which is exposed upward from the traveling frame , an accommodating portion formed in the traveling frame and configured to accommodate a lower half portion of the holding shaft ; wherein the movable cover has a through hole through which the upper half portion of the holding shaft can vertically extend and the through hole has a size which can displace the movable cover substantially only vertically with respect to the upper half portion , characterized in that each of the plurality of holding devices also comprises a shaft holding unit configured to hold the lower half portion accommodated in the accommodating portion , such that the lower half portion can slide in only a direction substantially perpendicular to the axis of the holding shaft , the shaft holding unit includes a slider formed on the lower half portion , and a guide portion configured to guide the slider such that the slider can slide in the direction perpendicular to the axis of the holding shaft , the upper half portion extends upward from the movable cover through the through hole , and has a spring receiving portion configured to receive one end of a compression coil spring in an upper end or in the vicinity of the upper end, the compression coil spring intervenes between the spring receiving portion and an upper wall of the movable cover , and at least one of the plurality of holding devices includes a shaft displacement amount detector configured to detect a horizontal slide displacement amount of the lower half portion of the holding shaft with respect to the traveling frame . a holding shaft formed by a single rod-like member having an axis which vertically extends with respect to the traveling frame , and having an upper half portion which is exposed upward from the traveling frame , an accommodating portion formed in the traveling frame and configured to accommodate a lower half portion of the holding shaft ; wherein the movable cover has a through hole through which the upper half portion of the holding shaft can vertically extend and the through hole has a size which can displace the movable cover substantially only vertically with respect to the upper half portion , characterized in that each of the plurality of holding devices also comprises a shaft holding unit configured to hold the lower half portion accommodated in the accommodating portion , such that the lower half portion can slide in only a direction substantially perpendicular to the axis of the holding shaft , the shaft holding unit includes a slider formed on the lower half portion , and a guide portion configured to guide the slider such that the slider can slide in the direction perpendicular to the axis of the holding shaft , the upper half portion extends upward from the movable cover through the through hole , and has a spring receiving portion configured to receive one end of a compression coil spring in an upper end or in the vicinity of the upper end, the compression coil spring intervenes between the spring receiving portion and an upper wall of the movable cover , and at least one of the plurality of holding devices includes a shaft displacement amount detector configured to detect a horizontal slide displacement amount of the lower half portion of the holding shaft with respect to the traveling frame . <EOS>