A number of wireless systems have recently adopted an adaptive modulation scheme to improve frequency efficiency. Unlike mobile systems, FWA (fixed wireless access) systems including IEEE 802.16 systems used for MAN (metropolitan area network) are allowed to use quasi-millimeter and millimeter bands, but those are vulnerable to rain attenuation. Adaptive modulation scheme is effective in those systems to improve the availability of wireless links. A 26 GHz-band broadband FWA system with a five-level (1024 QAM to BPSK) adaptive modulation scheme was proposed and developed by the authors for field experiments. This paper introduces field experimental results concerning switching thresholds for adaptive modulation controls using the developed FWA system. The numerical results reveal that conventional ideas, in which identical switching thresholds in terms of BER (bit error rate) apply to all modulation methods and switching thresholds maximizing Layer-2 throughput, can neither obtain the maximum TCP (transmission control protocol) throughput nor satisfy the upper limit of the UDP (user datagram protocol) packet loss rate.