An onset length changing device includes at least one processor configured to: perform a process of advancing a first vocal generation start timing of a first vowel in a certain syllable including a first consonant and a second vocal generation start timing of a second vowel in a syllable including a second consonant that is different from the certain syllable, based on a parameter designated in response to one user operation; and not to change a vocal generation start timing of a vowel in a syllable without including a consonant.