A speech synthesizer includes a memory configured to store a plurality of sentences and prior information of a word classified into a minor class among a plurality of classes with respect to each sentence, and a processor configured to determine an oversampling rate of the word based on the prior information, determine the number of times of oversampling of the word using the determined oversampling rate and generate sentences including the word by the determined number of times of oversampling. The plurality of classes includes a first class corresponding to first reading break, a second class corresponding to second reading break greater than the first break and a third class corresponding to third reading break greater than the second break, and the minor class has a smallest count among the first to third classes in one sentence.