This is a compatible headphone unit for the WonderSwan, WonderSwan Color, and WonderSwan Crystal. A headphone unit is required to play music in stereo. The case is a 3D-printed compatible part. While perfect operation with all software cannot be guaranteed, it has been confirmed to work without issues with major titles such as Final Fantasy and Digimon. (Operation confirmed) The connection terminals on the main unit are prone to poor contact due to aging. It is recommended to clean them first for better recognition. If you previously expressed interest in purchasing a power supply unit or case set, or if you have purchased one previously, please contact me for a discount.