An automatic clutch device includes an axial force generating mechanism formed by an electric motor adjacent to the outer periphery of an end of an input shaft of a transmission, and a rotation-linear motion conversion mechanism for converting the rotation of the rotor of the electric motor to a linear motion of a release bearing. The rotation-linear motion conversion mechanism includes plural tubes having different diameters and slidably fitted one in another to form a telescopic tube assembly on the input shaft; and cam mechanisms each disposed between a respective opposed pair of the tubes for converting the rotation of the larger-diameter one of the pair of tubes to a linear motion of the smaller-diameter one of the pair.