The objective of this study was to investigate the acquisition process of reading ability during preschool and 1st grade years, especially, to investigate longitudinally the relationship between subcomponents which composed the reading ability. Children were tested about their reading ability and subcomponents of it, e. g., decoding skill and working memory capacity for four consecutive times: June and January in their kindergarten and June and February in their 1st grade. The following results were confirmed: First, the earlier they can read letters, the more quickly they can read words but not non-words. Second, the performance of listening comprehension task were explained by working memory capacity and vocabulary. Third, their reading ability was explained by listening comprehension performance and the efficiency of decoding words. The conditions which constrain their reading comprehension at this age level were discussed.