Acquiring knowledge about the timing and speed of hand gestures is important to learn physical skills, such as playing musical instruments, performing arts, and making handicrafts. However, it is difficult to use devices that dynamically and mechanically control a user's hand for learning because such devices are very large, and hence, are unsuitable for daily use. In addition, since groove-type devices interfere with actions such as playing musical instruments, performing arts, and making handicrafts, users tend to avoid wearing these devices. To solve these problems, we propose PossessedHand, a device with a forearm belt, for controlling a user's hand by applying electrical stimulus to the muscles around the forearm of the user. The dimensions of PossessedHand are 10 x 7.0 x 8.0 cm, and the device is portable and suited for daily use. The electrical stimuli are generated by an electronic pulse generator and transmitted from 14 electrode pads. Our experiments confirmed that PossessedHand can control the motion of 16 joints in the hand. We propose an application of this device to help a beginner learn how to play musical instruments such as the piano and koto.