1. A text-to-speech system comprising a processing circuitry coupled to a memory, the processing circuit being configured to: receive an input text which contains a peculiar expression representing an expression not used in normal expressions; identify a position of the peculiar expression in the input text based on a normalization rule in which the peculiar expression, a normal expression for expressing the peculiar expression in a normal form, a non-linguistic expression style of the peculiar expression representing a manner in which the peculiar expression is read aloud, and a first cost are associated with one another, so as to generate one or more normalized texts; calculate one or more combinations of one or more positions to which one or more normalization rules are to be applied; calculate a total of the first cost or first costs in the case of applying the normalization rules for each combination of the combinations; normalize the input text based on the normalization rules by using the combinations for which the total is smaller than a first threshold value; perform language processing with respect to each of the normalized texts, and select a single normalized text based on result of the language processing; generate a series of phonetic parameters representing phonetic expression of the single normalized text; modify a phonetic parameter in the normalized text corresponding to the peculiar expression in the input text based on a phonetic parameter modification method according to the normalization rule of the peculiar expression; and output a phonetic sound which is synthesized using the series of phonetic parameters including the modified phonetic parameter.