A bearing device includes a piston arranged in a cylinder of a rotor, and a half bearing mounted on the piston to support a roller. The piston includes a recessed holding surface, and holding side surfaces on both axial sides thereof. Each holding side surface includes a ridge portion having an arc-shaped or elliptic-arc-shaped profile protruding toward a radially inner side of the piston. The half bearing includes a partially cylindrical portion, and protruding portions at a circumferential center thereof. Each protruding portion has a protruding portion end face which includes a central recessed surface, and two support recessed surfaces located on both circumferential sides thereof, that have an arc shape or an elliptic arc shape to correspond to the profile of the ridge portion, so that only the two support recessed surfaces are in contact with the ridge portion.