The invention provides a body motion detector that allows a user to check whether he/she makes motion with appropriate motion intensity for every motion thereby to obtain an excellent exercise effect while exercising such as walking and running. While a user makes motion, a CPU determines whether the user makes appropriate motion by the amplitude, the period, and the detection frequency of an acceleration signal inputted from an acceleration sensor unit, and when it is determined that the user makes appropriate motion, operates an alarm generator thereby to notify the user that he/she makes motion with appropriate motion intensity.