A remote elder care is important but it is difficult to support their correct dosing. We developed a remote elder care support system, which enables users to synchronize handwriting, and photos in real-time on ordinal smartphones and tablet devices. However, a caregiver was hard to check care receiver's dosing timely. And, it is undesirable to attach additional sensors to ordinal devices to reduce caregiver's load. To solve the problem, we developed a motion detector based on the attenuation of Bluetooth signals, the 2.4 GHz band. The advantages of our method are that equipment of any devices to detect care receivers is unnecessary and non-expert can sets the system flexibly. We experimented the attenuation of Bluetooth signals by a human body, thus we concluded that Bluetooth could be used for a motion detector.