Subtitle translation often involving a limitation of time and space for the target rendition is called “constrained translation” where character count (i.e. the number of words) permitted in a segment is restricted. This is also known as a “6 second rule” (or 12 characters per second) in the case of English subtitle. Japanese subtitling is also confined with a similar rule that allows up to only four Japanese characters per second of utterance. These restrictions are necessary for ease of understanding or readability of subtitles; however, this over 85-year-old rule of Japanese character-restriction has never been tested scientifically as to its validity. Thus, the authors have carried out an experiment to examine the validity of the four-character-per-second rule by investigating the audiences’ reception of subtitles with the use of an eye-tracker. The result shows that up to 6 characters per second is accepted without distracting spectators’ focus and comfort.