2.4 GHz-band wireless LAN system based on a new standard, IEEE 802.11g, has been taking a great attention as it provides the attractive features such as low cost, unlicensed spectrum use, and high speed transmission rate up to 54 Mbps. However, 2.4 GHz radio frequency band is also used for Industrial, Scientific and Medical (ISM) devices such as microwave ovens, and the man-made noise leaked from ISM devices is known to be one of the major causes of the degradation in the performance of wireless communications systems using 2.4 GHz radio frequency band. In this paper, we evaluate the bit error rate (BER) and the throughput performances of WLAN system based on IEEE 802.11g standard (IEEE 802.11g WLAN system) under man-made noise environment, and discuss the effect of man-made noise on the performance of IEEE 802.11g WLAN system. Numerical results show that the BER and the throughput performances of IEEE 802.11g WLAN system are much degraded by the influence of man-made noise.