16. A speech correction apparatus comprising: a speaker for generating speech to be corrected; a microphone set at a hearing position; a signal separating unit comprising: a filter having an acoustic characteristic as an impulse response of an acoustic system between the speaker and the microphone; and an operating unit for subtracting a signal which is obtained by passing a signal supplied to the speaker according to the speech to be corrected through the filter from a signal received from the microphone when the speech to be corrected is generated by the speaker, wherein a signal corresponding to the speech to be corrected is supplied by the filter and a signal corresponding to the ambient noise is supplied from the operating unit, so that the ambient noise is separated from the speech to be corrected at the hearing position; a coefficient update unit for updating a filter coefficient of the filter so that the power of the signal supplied from the operating unit is minimized; a filter initial setting unit for setting an initial value of the filter coefficient, which is updated by the coefficient update unit, at a value other than 0; and a speech correcting unit for correcting a sound pressure level of the speech to be corrected generated by the speaker based on a comparison between a time length Ls for calculating an average power of the speech to be corrected and a time length Ln for calculating an average power of the ambient noise, which are separated by the signal separating unit.
16. A speech correction apparatus comprising: a speaker for generating speech to be corrected; a microphone set at a hearing position; a signal separating unit comprising: a filter having an acoustic characteristic as an impulse response of an acoustic system between the speaker and the microphone; and an operating unit for subtracting a signal which is obtained by passing a signal supplied to the speaker according to the speech to be corrected through the filter from a signal received from the microphone when the speech to be corrected is generated by the speaker, wherein a signal corresponding to the speech to be corrected is supplied by the filter and a signal corresponding to the ambient noise is supplied from the operating unit, so that the ambient noise is separated from the speech to be corrected at the hearing position; a coefficient update unit for updating a filter coefficient of the filter so that the power of the signal supplied from the operating unit is minimized; a filter initial setting unit for setting an initial value of the filter coefficient, which is updated by the coefficient update unit, at a value other than 0; and a speech correcting unit for correcting a sound pressure level of the speech to be corrected generated by the speaker based on a comparison between a time length Ls for calculating an average power of the speech to be corrected and a time length Ln for calculating an average power of the ambient noise, which are separated by the signal separating unit. 17. A speech correction apparatus according to claim 16 , wherein the initial value set by the filter initial setting unit is a filter coefficient corresponding to a representative car model, which is determined based on the statistical number of cars for sale.