Abstract: In order to make it possible to measure the moisture content of coal on an on-line basis with a high degree of precision, a moisture content measuring system including a sample container for holding a sample, a weighing instrument having a turntable adapted to rotate with the sample container placed thereon, an A/D converter responsive to a signal from the weighing instrument, a calculator responsive to a signal from the A/D converter for calculating the moisture content of the sample, an indicator responsive to a signal from the calculator and a microwave generating and irradiating apparatus for heating the sample, is improved. The improvements reside in that the system includes a hot gas generator for heating the sample with hot gas at a low temperature, and a dehumidified dry air circulating means for removing moisture from the sample.