PURPOSE: To provide the method and means to control the rate of composition change of a fluid having its changing composition especially in the case of mixed fuel that is used for an alternate fuel automobile.

CONSTITUTION: A composition change controller 10 has a reservoir 26 having a top wall 28, a bottom wall 30 and a side wall 32 which prescribes the internal capacity 34 of the controller 10. The reservoir 26 has a fluid receiving means 36, a means 38 which mixes the received fluid with the fluid that is already stored in the reservoir 26, a means 40 which shows the instantaneous average composition of the fluid stored in the reservoir 26 based on the composition of the fluid that is drawn out of the reservoir 26 and partly draws the fluid stored in the reservoir 26, and a means 42 which discharges the fluid drawn out of the reservoir 26 to a desired object placed at the downstream side. In such a constitution, the composition of the fluid discharged from the reservoir 26 has a rate of change smaller than that of the fluid received by the reservoir 26.