A chuck device provided with a loosening prevention mechanism, in which claws threadedly engaged with a rotary nut are moved back and force to be opened and closed by the rotation of the rotary nut provided on a body so that a tool is clamped by the claws. A receiving portion is provided to face a rear surface of the rotary nut, rolling members are provided between the receiving portion and the rotary nut, the rear surface of the rotary nut and/or a front surface of the receiving portion is formed to be slanted surfaces expanding to the outside, and surfaces on which the rolling members are moved outwardly to contact when the tool is clamped by the claws and a fastening reactive force in a thrust direction is applied to the rolling members are formed into a movement suppressing surface for suppressing movement of the contacted rolling members.