A musical tone generating apparatus is provided for a keyboard electronic musical instrument. The apparatus consists of a musical tone source, a coefficient generator, a digital filter and so on. The coefficient generator generates at least one time dependence coefficient, and applies It to output of the digital filter. The parameter controller changes at least one parameter, which designates tone color of the musical tone, in accordance with the time dependent coefficient to thereby realize time-variant changing rate of envelope of musical tone. And, the coefficient generator and the parameter controller are used by means of time sharing technique. That is, the coefficient is used as plural units, wherein each unit generates a coefficient in each stage, and the digital filter is also used as plural units. Each digital filter unit changes a parameter of tone color of the musical tone in accordance with the coefficient supplied thereto.