In manual assembly work, parts are often assembled by applying force with a simple tool or by hand. A worker thus needs control the force he or she applies in working, as an appropriate level of force is requisite for minimizing work failures and improving efficiency. The object of this study is to clarify the relationship between the level of force and level of training difficulty in manual assembly work. Measurements of the training difficulty for 10 test subjects (persons being tested) at force gauges of 30, 40, 50, 60, 70, 80, 100, and 120N revealed a relationship between the force gauge internal to each test subject and the training difficulty.