[en] Since the Chernobyl-4 accident, the importance of void reactivity has heightened. The Chernobyl-4 accident was reactivity accident due to the rabid increase of void ratio in the coolant, accordingly the evaluation of the safety peculiar to nuclear reactors and the behavior at the time of severe accidents has become to be carried out. In particular, it is the present state that in the pressure tube type reactors using coolant and moderator separately, attention is paid to the nuclear safety. In boiling light water-cooled pressure tube type heavy water reactor ATR, the coolant void reactivity when the void ratio in coolant increased or loss of coolant accidents occurred can become positive or negative according to such lattice condition as fuel composition, because the state of moderator does not change. Therefore, to suppress void reactivity as far as possible has become an important subject for improving the operation control and nuclear safety of reactors. Among the reactor physics research using the DCA, the knowledge obtained by the measurement of coolant void reactivity in an ATR lattice using substitution method and its analysis is reported. The role of coolant void reactivity in criticality experiment, the experiment and its results are described. It was found that by the present method, the reactor physics information peculiar to a test lattice can be extracted with a small number of test fuel. (K.I.)