With a aim at measuring heat capacities of tiny crystals weighing about 1 mg, we have constructed a thermal relaxation type calorimeter available at low-temperatures. The addenda part consists of a small chip-type ruthenium oxide thermometer and a film heater of which heat capacity is about 40 nJ/K at 1 K. This sample cell is equipped with a 3He refrigeration system whose cooling power is controllable by adjusting the absorption rate of charcoal sorb located at low-temperature part. The attained minimum temperature by this refrigeration system was 289 mK. The heat capacity data for 0.69 mg Al sample obtained by this calorimeter is presented. We are going to use this calorimeter for the thermodynamic studies of several organic conductors and molecular based magnetic materials.
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