The invention provides a speech synthesis apparatus which can produce synthetic speech of a high quality with reduced distortion. To this end, upon production of synthetic speech based on prosodic information and phonological unit information, the prosodic information is modified using the phonological unit information, and duration length information and pitch pattern information of phonological units of the prosodic information and the phonological unit information are modified with each other. The speech synthesis apparatus includes a prosodic pattern production section for receiving utterance contents as an input thereto and producing a prosodic pattern, a phonological unit selection section for selecting phonological units based on the prosodic pattern, a prosody modification control section for searching the phonological unit information selected by the phonological unit selection section for a location for which modification to the prosodic pattern is required and outputting information of the location for the modification and contents of the modification, a prosody modification section for modifying the prosodic pattern based on the information of the location for the modification and the contents of the modification outputted from the prosody modification control section, and a waveform production section for producing synthetic speech based on the phonological unit information and the prosodic information modified by the prosody modification section using a phonological unit database.