In calorimetry, the heat concerned is necessarily accompanied with incidental heat due to the irreversibility of measurement process. Hence, the reduction of such incidental heat is a key technology to design calorimeters of the lowest uncertainty. An airtight ampoule breaker and a low-friction stirrer were designed to reduce the incidental heat due to the vaporization of solvent and the friction of stirring process that are presumably two of major sources of the incidental heat in measurement of heat of immersion. Testing a calorimeter adopting these devices, the fluctuation of base line was found to be as low as the level of the characteristic noise of amplifier. The results of two repetitive measurements of Joule heating in the measurement vessel coincided within 0.57 %.