Seal device and construction machine

A seal device comprising:

• a first member having an inner circumferential surface;
• a second member rotating relative to the first member, the second member facing the first member in a rotation axis direction;
• a third member rotatable relative to the first member, the third member being received in the first member;
• a seal ring having:
• an outer circumferential surface that faces the inner circumferential surface in a radial direction and is spaced apart from the inner circumferential surface;
• a back surface in contact with the first member;
• and
• a seal surface in sliding contact with the second member;
• an elastically deformable elastic ring disposed between the inner circumferential surface of the first member and the outer circumferential surface of the seal ring;
• wherein the first member is formed in an annular shape with an opening,
• wherein the second member is a cover provided at an end of the third member in the rotation axis direction,
• wherein the cover closes the opening of the first member, and
• wherein the elastic ring presses the seal surface of the seal ring against the second member to seal between the seal surface and the second member.