A fuser device includes a belt part rotating in a tension free state to carry a medium in a medium carrying direction, a first nip forming part arranged inside the belt part, a second nip forming part arranged outside the belt part sandwiching the belt part with the first nip forming part so that a nip area is formed therebetween, and a regulation member regulating a movement of the belt in a width direction of the belt part. The regulation member is configured with a belt regulation part and a slant part. The belt regulation part has a flat shape arranged near one of side edges of the belt part along the nip area with a predetermined gap (W6). The slant part extends from an edge of the belt regulation part.