1. A communication apparatus mounted with a speech speed conversion device, comprising: a compressor that compresses telephone call voice which is transmitted from another communication apparatus; a message preserver that preserves the telephone call voice, which is compressed by the compressor, as a message; an expander that expands the telephone call voice which is preserved in the message preserver; a speech speed convertor that includes a voice buffer which temporarily maintains the telephone call voice expanded by the expander, and that performs speech speed conversion on the telephone call voice which is read from the voice buffer, and outputs resulting voice from a speaker; and a monitor that temporarily stops to expand the telephone call voice in the expander in a case where the monitor determines that an idle capacity of the voice buffer approaches a predetermined lower limit value.