1. A speaker speed conversion method for converting the speed of speech that is received as input, said method comprising: a risk site detection step of detecting sites of risk regarding sound quality from among speech that is received as input; a frame boundary detection step of detecting a plurality of points that can serve as candidates of frame boundaries from among speech that is received as input, and from among these points, supplying as a frame boundary the point that is predicted to be best in terms of sound quality; and an OLA (overlap and add) step of performing speed conversion based on the detection results of said frame boundary detection step; wherein said frame boundary detection step eliminates, from candidates of frame boundaries, sites of risk regarding sound quality that were detected in said risk site detection step, and at least one of the risk site detection step, the frame boundary detection step, and the OLA step is performed by a computer.