  We have developed a Monte-Carlo simulation code for an aerogel \v Cerenkov
Counter which is operated under a strong magnetic field such as 1.5T. This code
consists of two parts: photon transportation inside aerogel tiles, and
one-dimensional amplification in a fine-mesh photomultiplier tube. It simulates
the output photoelectron yields as accurately as 5% with only a single free
parameter. This code is applied to simulations for a B-Factory
particle-identification system.
