1. A construction method for a speech recognition model, comprising the steps, executed by a computer system, of: acquiring alignment between speech of each of a plurality of speakers and a transcript of the speaker; automatically joining transcripts of the respective ones of the plurality of speakers along a time axis, creating a transcript of speech of mixed speakers obtained from synthesized speech of the speakers, and replacing predetermined transcribed portions of the plurality of speakers overlapping on the time axis with a unit which represents a simultaneous speech segment; and constructing at least one of an acoustic model and a language model which make up a speech recognition model, based on the transcript of the speech of the mixed speakers.
8. The construction method according to claim 1 , wherein the creating step includes a step of creating an acoustic model transcript of the speech of the mixed speakers and a step of creating a language model transcript of the speech of the mixed speakers; and the step of creating the acoustic model transcript of the speech of the mixed speakers comprises the steps, executed by the computer system, of: determining which speaker is dominant in predetermined transcribed portions of the plurality of speakers overlapping on the time axis, and adopting, in response to determining that any speaker is dominant, a transcribed portion corresponding to the speaker determined to be dominant instead of replacement with the unit which represents the simultaneous speech segment.