Abstract: A fastening force auxiliary device (1) includes a seat part (10) in which a hole (11) allowing a screw part (3) of a miniscrew (2) to be inserted through is formed and which receives a head part (5) of the miniscrew (2). Further, the fastening force auxiliary device (1) has three spiked portions (13) extending from a periphery of the seat part (10) in a driving direction of the miniscrew (2). The spiked portions (13) extend so as to spread outward from the periphery of the seat part (10) and so as to gradually taper off. When the miniscrew (2) is implanted and fixed in a jaw bone or the like in an oral cavity, it is screwed into the jaw bone or the like while the screw part (3) is inserted through the hole (11) of the fastening force auxiliary device (1). Consequently, a flange (7) is received by the seat part (10) and tips of the spiked portions (13) come into pressure contact with a surface of the jaw bone or the like by being pressed by the flange portion (7).