A pipe coupling device includes a joint body, a C-shaped retaining ring that includes a flange part radially protruding from an outer peripheral surface of the retaining ring, a pressing plate that includes a recessed part configured to house the flange part, and includes a front wall configured to face a rear end of the flange part. The joint body, the retaining ring, and the pressing plate are coupled together in an axial direction. The flange part has a substantially rectangular shape extending along the axial direction, and an inclined surface is formed on at least one of the rear end and the front wall. An inner end of the inclined surface is positioned closer to the joint body than an outer end thereof. The inner end is closer to a center axis of the joint body than the outer end. A gap is formed between the rear end and the front wall.