PURPOSE: To simplify and miniaturize whole mechanism and to improve the durability of a whole automatic extension device by providing an intermediate gear part so that it can move with respect to a supporting shaft and operating elastic arm members provided in a limit switch by means of the switch operation parts of a gear supporting frame.

CONSTITUTION: The intermediate gear part consisting of a warm wheel 2 and a small gear 3 is provided so that it can slide with respect to the supporting shaft 9 provided in a case 10 with the gear frame 4. The switch operation parts 14 and 14' are provided so that they are confronted with in the gear frame 4, and the elastic arm members 5 and 5' are provided between said parts 14 and 14'. Armatures 6 and 6' which can be inverted are fitted on the sides of play ends in the members 5 and 5' that are bent in U-shapes. The elastic arm members 5 and 5' represent induction pressure in the limit switch, and the switch is operated by the driving force of the warm wheel 2 which a warm 1 drives. Consequently, an off operation with respect to motor driving can be obtained by using spring pressure without the necessity of a special clutch device.