The purpose of the present study is to examine that (1) are there breaking points in mental number line in Japanese children, and (2) does it relate children’s calculation performances. The magnitude decision task for eight number pairs, e.g. “3-5”, was used in this study. Participants were 40 first graders children. An ANOVA for the decision time revealed that the main effect of number pairs is significant (F(1, 39) = 8.45, p<.01). A Tukey’s HSD test indicated that the decision time for “4-6” and “5-7” were slower than for “3-5”, and the time for “9-11” and “10-12” were slower than for “8-10”. These results indicated that there are the five-break and the decade-break in metal number line in Japanese children. And a negative correlation between the decision time and the performances of calculation problem were obtained. It suggested that the calculation skill is acquired based on the mental number line.