The present study was designed to examine the reasons of senior high school athletes' keeking in or dropping out of their sport team activities. The data was obtained by questionnaries distributed to 550 keep-ups and 291 drop-outs, and Hayashi's quantification theory II was applied to the data. Main findings are as follows: 1. The most influential items for dropping out are "conflicts and disagreements with coaches or teammates", "having other things to do" and "being incompatible with study" for males, and "injuries","conflicts and disagreements with coaches or teammates" and "having other things to do" for females. 2. Besides the items stated above, the most influential item for dropping out, for both males and females, is such a self-panishing one as "weakness of one's own will". 3. The most influential factor of determining keeping in or dropping out is their "daily sport activities" which consists of whether or not they are regular members, they perceive their training hard, they experience impression, and they like their coaches. 4. In case of females, the factor "family environment" determines keeping in or dropping out more strongly than the factors which concern to their own. This is supposed to be the reflection of female's dependent and conformable psychological traits. 5. Males who perceive their team activities hard but are satisfied with coaches, are regular members, and have less study time, and females who acknowledge their sport team activities in their school active, perceive their team activities are not so hard, experience more impression, are satisfied with coaches, are starters, and have less achievement motives tend to be promoted to keep in their sport team activities.