In this paper, the authors introduce a stand-alone synthesizer, ``B.O.M.B. -Beat Of Magic Box --" for electronic music sessions and live performances. ``B.O.M.B." has a wireless communication system that synchronizes musical scale and tempo (BPM) between multiple devices. In addition, participants can change master/slave role between performers immediately. Our primary motivation is to provide musicians and nonmusicians with opportunities to experience a collaborative electronic music performance. Here, the hardware and interaction design of the device is presented. To date, numerous collaborative musical instruments have been developed in electronic music field [1][2][3]. The authors are interested in formations of musical sessions using stand-alone devices and leader/follower relationship in musical sessions. The authors specify three important requirements of instrument design for musical session. They are as follows: (1) Simple Interface: Interface that enables performers to control three sound elements (pitch, timbre, and amplitude) with simple interaction. (2) Portable Stand-alone System: System that runs standalone (with sound generators, speakers, and butteries). Because musical sessions can be improvised at any place and time, the authors consider that portability is essential in designing musical instruments for sessions. (3) Wireless Synchronization: System that supports ensembles by automatically synchronizing tempo (BPM) and tonality between multiple devices by air because of portability. In addition, performers can switch master/slave roles smoothly such as leader/follower relationship during a musical session. The authors gave ten live performances using this device at domestic and international events. In these events, the authors confirmed that our proposed wireless synchronization system worked stable. It is suggested that our system demonstrate the practicality of wireless synchronization. In future, the authors will evaluate the device in terms of its stability in multi-performer musical sessions.