A rotary damper 1 includes a synthetic resin-made accommodating case 4 for accommodating in its interior 2 a viscous fluid 3 which is constituted by silicone oil or the like and whose viscosity declines with a temperature rise, and a synthetic resin-made rotor 9 which is disposed in the interior 2 of the accommodating case 4 rotatably, i.e., so as to be rotatable in R1 and R2 directions about an axis O, and which partitions the interior 2 of the accommodating case 4 into at least two chambers, i.e., in this embodiment, two chambers consisting of chambers 5 and 6 and two chambers consisting of chambers 7 and 8, in cooperation with the accommodating case 4.