  Effect of randomness in the double-exchange model is studied. Large
fluctuations and spatial random distribution of impurities are taken into
account in an essentially exact manner by using the Monte Carlo calculation.
The randomness suppresses the ferromagnetism by reducing the coherence of
itinerant electrons. The suppression is significant in the critical region
where the fluctuations are dominant. Temperature dependences of the
magnetization are estimated for finite-size clusters. A characteristic
temperature for phase transition $T^{*}$ is estimated from the inflection
point, which is expected to give a good approximation for the critical
temperature in the thermodynamic limit. Our results suggest that the
ferromagnetism becomes unstable more rapidly than predicted in the previous
theoretical results by the coherent-potential approximation.
