1. A voice authentication apparatus comprising: hardware circuitry having: a characteristics analysis part that analyzes characteristics of a sample noise which is generated around a subject while the subject generates a sample voice for authentication of the subject; a first setting part that sets a first correction value according to a first relation between the sample voice and the sample noise analyzed by the characteristics analysis part and a second relation between a reference voice which has been previously registered and a reference noise which is generated during registration of the reference voice, such that the first correction value is set according to both a level ratio between the reference voice and the reference noise and a level ratio difference between the level ratio between the reference voice and the reference noise and a level ratio between the sample voice and the sample noise; a correction part that corrects an index value, which indicates a degree of similarity between a feature quantity of the reference voice and a feature quantity of the sample voice obtained from the subject, based on the first correction value set by the first setting part; and a determinator that determines authenticity of the subject by comparing the index value corrected by the correction part with a predetermined threshold value.