A coupling device including a first rotary member and a second rotary member are opposed and connected to each other on the same axial line, wherein a cap to cover a connection part is a cylindrical shape having a bottom wall and a protrusion insertion hole formed on the bottom wall and covers the first rotary member from a connection end-side of the first rotary member, wherein the cap is axially urged by an elastic member to bring at least one of an opening-side end face and the bottom wall of the cap into contact with an annular end face formed on the first rotary member, and wherein an accommodation chamber of the connection part is formed in the cap to reserve lubricant, and an exit of the accommodation chamber of the connection part is blocked or narrowed by a force of the elastic member.