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 under a favorable condition. Its transfer between the conformational and thermal degrees of freedom allows the entropy of deformation to be determined quantitatively for the polymer material. The explicit change of the mode of entropy is useful in introduction of the entropy concept in thermodynamic courses. Construction of the apparatus and its operation are described in detail for those who will 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.