A new experimental technique for the measurement of unstable solid density that uses an oscillating U-tube was proposed. The technique was based on the simultaneous lever rules for the mass and the volume of two phase, solid + liquid (molten solid), in the U-tube. By means of this technique, the solid density can be easily determined from the overall density of solid + liquid with the combination of the literature phase diagram. To ensure the reliability, density of an amphiphilic congruent melting compound, tetrahydrofuran clathrate (THF . 17H(2)O), was measured at 276.37 K under the atmospheric pressure. The density of the hydrate was estimated to be 998.40 kg/m(3). The value was very close to the theoretical one, 999.14 kg/m(3) This technique needs a phase diagram of clathrate though it is very useful because of its simplicity and easiness to expand to a wide range of temperatures and pressures.