Provided is a torsion tester which comprises a sealed case accommodating a decelerator, and an oil cup. An internal space of the case and an internal space of the oil cup communicate with each other through a communication passage. A lubricant is filled into the case. When the lubricant in the case is thermally expanded, the surplus lubricant moves from within the case to the oil cup through the communication passage to be accumulated. When the volume of the lubricant in the case is decreased, the lubricant moves from the oil cup to within the case through the communication passage to fill the shortage so that the case always remains filled with the lubricant.
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