To determine the minimal amount of water necessary for survival in Bacillus subtilis, bound and free water were measured by time domain reflectometry (TDR) and the gel-sol phase transition of bound water was measured by differential thermal analysis-thermogravimetry (DTA-TG). Both the vegetative and the spore form of Bacillus subtilis were dried at different temperatures. From the measurements, the relationship between survival of the bacteria and water can be summarized as follows. (1) The vegetative form of bacteria was still 100% viable even when 80% of bound water was removed. (2) The spore form contained only 20% of bound water, which was easily and quickly removed by drying, and resulted in loss of viability. (3) Phase transition between gel and sol phases of bound water occurred repeatedly in the dried vegetative 100%-viable bacteria, which still had 20% bound water. The bacteria were still viable after such measurements. (4) In the spore form, phase transition was not detected.