A fast and powerful down picking skill of rhythm guitar is crucial for heavy metal songs. At more than 400 beats per minute, which is a typical starting point for heavy metal songs, most players feel hard to keep the picking rhythm. In this paper, we propose a wrist-worn inertial motion tracking device for analyzing the guitar picking motion. A population test showed that a wrist twist angular velocity signal would give sufficient information on the picking timing to evaluate the accuracy of rhythm. It was also found that the power of sound had a strong correlation with the wrist twist angle and the upward linear acceleration signals. We developed a picking evaluation system by using characteristic motion parameters that informs the player of the speed, power, and smoothness of the picking motion. As a result, five out of eight beginners to play heavy metal guitar reached the speed of 400 beats per minute in a short training period.