A speech synthesis unit, in which a natural speech is cut and taken out separately in predetermined time intervals, the acoustic characteristics and parameters in the predetermined time interval are abstracted and then speech is synthesized based on these characteristics and parameters. The speech synthesis unit alone can process different quantities of the acoustic parameter information. For this purpose, the time interval of one analysis frame is changed so as to change the information quantity per unit of time, without any variation of the number of bits of the characteristics and parameters distributed in one analysis frame, and the time interval of one synthesis frame in the synthesizing unit is correspondingly changed so as to match the time interval of one analysis frame with the time interval of one synthesis frame.