The rhythm of vocalizing a written language depends on a merge process that combines meaningless linguistic units into a meaningful lexical unit, word, or Bunsetsu in Japanese. However, in most previous studies, written language was presented to the participants in lexical units (word-by-word) with explicit inter-word (or inter-Bunsetsu) marks or spacing. Therefore, it has been difficult to conduct psychophysical assessment of the participants’ own speed in segmenting meaningful units from unstructured written language when reading. Here, we hypothesized that the spontaneous reading speed of Japanese readers reflects their own punctuation process, even when sentences are written without punctuation marks or spaces. To test this hypothesis, we developed a new “self-paced sequential letterstring reading task,” which visually presents sentences letter-by-letter. The task required participants to push a button to proceed to the next letter at their own pace, hence allowing evaluation of the reaction time (RT) to individual letters. We found that the average RT decreased parametrically as the position of the letter approached the end of a Bunsetsu. Moreover, the RT increased drastically at the last letter completing the Bunsetsu. Participants were not shown any punctuation marks and not instructed to explicitly recognize the punctuations during reading. Therefore, these effects strongly suggest that the implicit and spontaneous punctuation is the origin of the rhythm in reading. These results show that spontaneous punctuation of letterstring affects the reading speed. The task we have developed is a promising tool for revealing the temporal dynamics of natural reading, which opens a way to shape the fluency of script-to-speech human interfaces.