Many reports indicated that children with developmental dyslexia have distinctive disorder in reading ability, however, there are no studies to investigate such ability based upon the characteristics of quantitative measurement in reading single character. Purpose of this study is to develop an effective method to evaluate the reading ability quantitatively. Specifically, we built a system to precisely measure the voice onset time (VOT) for hiragana characters. Here we report the results of VOT measurements in both normal and dyslexic populations. A total of eight children with dyslexia, eight healthy children and 15 healthy adults participated in the study on informed consent basis. Characters independently composed of syllabic sounds (non-contracted and contracted sounds, 106 characters in total) were chosen as the target data sets. The system automatically detected each pause and VOT and determined the time length of pronunciation in milliseconds. As a result, the time to pronounce a single character by the children with dyslexia was significantly longer than that by the healthy children and adults. The difference in VOT between non-contracted and contracted sounds suggested that the phonetic difference may affect the VOT. In conclusion, our proposed system can evaluate the reading ability for dyslexic children and it also demonstrated the effectiveness to organize the optimal strategy for improving their reading ability.