This paper is concerned with a computer simulation of active noise control using the boundary element method for analyzing three-dimensional acoustic field problems. The active noise control under consideration is reduced to an optimum problem to determine the optimal set of parameters defining the sound pressure of a secondary source to be attached. The optimal set of parameters are selected through the usual optimization procedure so that the sum of the sound pressure values measured at the duct exit should be minimal. Computation is carried out for typical examples in which the duct is embodied in the infinite plane and the noise through the duct is radiated to the semi-infinite acoustic field, whereby the potential usefulness of the computer simulation system developed is demonstrated.