A plurality of first viscoelastic bodies are disposed in insertion holes of a first rotary member. A plurality of second viscoelastic bodies are disposed in guide holes of a second rotary member. Each of the first and the second viscoelastic bodies has an elastic coefficient changing in accordance with aging due to use. A guide member moves a contact member along the insertion hole and the guide hole. The contact member applies a compressive load to the first viscoelastic body. The aging of the second viscoelastic body due to the compressive load applied thereto moves a position of the first viscoelastic body where the compressive load is received from the contact member toward a farther side from a rotational shaft side.