Temperatures, relative humilities, and water vapour pressures inside and outside of the test mattress were compared with those in case of cotton mattresses while resting in bed. The test mattress had a raft-structure made of 21 hard polyurethan-foam cylinders, 7.8 cm. in diameter, 90 cm, in length. The structure of the raft was covered with 3 sheets of layers, a sheet of soft polyurethan-foam, a sheet of polyethylene air-caps and cotton cloth, forming a sandwich-structure. The results were as follows; 1) When the microclimate inside of bed reached the comfortable and stady state, the values of temperature, relative humidity and vapour pressure in the space between the tatamimat and the mattress were smaller in case of resting in raft-structure mattress than in cotton mattress. 2) In case of resting in raft-structure mattress, moisture evaporation from a human body did not permeat the mattress, but diffused along the back of the body through the concave surface of air-caps. 3) As a result of observation over two years it was confirmed that wet moulds did not grow on the surface of either tatami-mat or the raft-structure mattress in case of resting in raft-structure mattress.