The purpose of the present invention is to provide a mouth-opening training device with which continuous rotational movement and anterior sliding movement of the temporomandibular joint can be performed smoothly, the device thereby being capable of guiding the movement of the mandible so as to trace an actual mandibular mouth-opening movement. The mouth-opening training device is provided with: an upper member, the base end of which is grasped and the tip of which can engage with a user's maxilla; a lower member, the tip of which can engage with the user's mandible and a portion of which is in contact with the upper member so as to allow sliding; and a connecting section for connecting the upper member to the lower member so as to allow sliding and allow opening and closing. As the tip of the upper member is opened with respect to the tip of the lower member, the lower member moves in the direction of the tip.