A rotational speed reduction device 1 for a truck tailgate includes a housing 5, a rotor 8 disposed inside the housing 5, a viscous fluid 14a accommodated in an inner space 13a, passages 15a and 16a provided in the rotor 8, a one-way valve 19 disposed in the passage 15a, and a one-way valve 20a disposed in the passage 16a.