1. A voice synthesis method for synthesizing a voice waveform to continuously change a feature of synthetic voice of a range assigned a predetermined identifier included in input text upon outputting synthetic voice corresponding to the text, comprising: a setting step of setting a desired range of text to be output, in which the feature of synthetic voice is to be continuously changed, using a predetermined identifier including attribute information that represents a change mode of the feature of synthetic voice; a recognition step of recognizing the predetermined identifier and a type of attribute information contained in the predetermined identifier from the text with the identifier, which is set in said setting step; and a voice synthesis step of synthesizing a voice waveform, whose feature of synthetic voice continuously changes, in accordance with the attribute information contained in the predetermined identifier, by interpolating synthetic voice corresponding to text within the desired range of the text with the identifier in accordance with a recognition result in said recognition step.