We have constructed an apparatus by which the temperature of a piece of rubber is measured while it is stretched or allowed to shrink. The length of the rubber piece and the tension on it are also measured. Resulting data represent the mechanocaloric effect in rubber in which the entropy behaves as a conserved quantity. Its transfer between the conformational and thermal degrees of freedom allows quantitative determination of the entropy of deformation to be made for the polymer material. The explicit change of the mode of entropy is useful in introduction of the entropy concept in thermodynamic course. Construction of the apparatus and its operation are described in detail for those who may take up the subject in a thesis for a bachelor’s or higher degree。 Possible uses of the mechanocaloric method in soft matter research are also discussed.