We have developed a method for synthesizing a singing voice by gradually changing the musical expression based on speech morphing. This paper shows the advantages of this method in comparison with the approach of binary discrete transformation between two expressions, confirmed by statistical analyses of perception tests. In order to synthesize different expressional strengths of a singing voice, a "normal" (without expression) voice of a particular singer is used as the base of morphing, and three different expressions, "dark," "whispery" and "wet," are used as the target. Through our experiments, we confirmed i) the proposed morphing algorithm effectively interpolates the expressional strength of a singing voice, ii) an approximate equation of the perceptual sense can be used to calculate the morph ratio at a perceptually linear interval, and iii) our gradual transformation method can generate a natural singing voice from the interpolation of two different expressions.