This paper describes advanced utilization of moisture in gas turbine systems for the purpose of improving thermal efficiency and output. We have proposed and been developing AHAT (Advanced Humid Air Turbine) system. The AHAT system is a system which uses existing mutual technologies and aims high performance equal to that of the HAT system. After many studies and elemental tests, a 4MW-class pilot plant was planned and built in order to verify feasibility of the AHAT system from the viewpoints of heat cycle characteristic and engineering. The gas turbine consists of a two-stage centrifugal compressor (pressure ratio of 8), a reverse-flow type single-can combustor, and a two-stage axial-flow turbine. The plant thermal efficiency exceeded 40%LHV.