Abstract: A pair of pressure plates and a spring unit situated between the pair of pressure plates. The spring unit includes plural springs. Each spring includes, in a longitudinally sectional shape, a curved central portion having substantially arc shape projecting in a direction perpendicular to an expansion/contraction direction, a pair of curved end portions each having substantially arc shape projecting at extensions of both ends of the curved central portion in a direction opposite to the projecting direction of the curved central portion, and base portions formed by extending edges of the curved end portions in substantially parallel with the direction perpendicular to the expansion/contraction direction. The base portions are respectively secured to the pair of pressure plates so the spring is located between the pair of pressure plates. The spring length in natural state has linear straight portions between the curved central portion and the curved end portions.