An example of an apparatus includes a ring-shaped portion, a detector and a main portion. At least a part of the ring-shaped portion is elastically deformable. The detector detects deformation of the ring-shaped portion. The main portion is secured to the ring-shaped portion. The main portion includes an attachment portion, a terminal and a transmitter. A game controller can be attached to the attachment portion. The terminal is for electrical connection with the game controller. The transmitter transmits data regarding a detection result of the detector to the game controller through the terminal.