Computer simulation is a useful technique for developing the active control system (referred to as the ANC system). In this paper, the technique is introduced of sound field analysis for an ANC system based on the boundary integral equation method (BIEM). Since the BIEM accounts for wave motion characteristics precisely, the dynamic behavior of the control sound field can be considered, especially in the low-frequency band. The objective of the study is to investigate a transient sound field by applying the BIEM to practical problems. In order to introduce the BIEM technique to actual situations, including boundary problems with the existing acoustic materials, a database was developed on the normal acoustic admittance ratio of the materials. The ordinary feedforward model of the ANC system was used to control the noise field in the computer simulations. The results, in which the transient sound fields controlled by the ANC system are shown, will be very useful in understanding how to build up the control sound field.