According to a rotary clamp cylinder of this invention, in a rod cover 5, attachment holes 18 are formed so as not to intersect an axis CL of a piston rod 11, and pin members 19 are rotatably fitted in the attachment holes 18, respectively. An amount L of engagement between each pin member 19 and each turning guide groove 14 becomes greater, on the side of the axis CL of the piston rod 11 in the pin member 19 than on the side opposite to the side of the axis CL of the piston rod 11, and the outer circumferential face 21 of each pin member 19 on the side opposite to the side of the axis CL of the piston rod 11 can be supported by the inner face of each attachment hole 18, over substantially the entire length thereof. Accordingly, excessively large load and/or impact could be received by the pin members 19, as such the increase of the transfer speed upon clamping or unclamping, and/or attachment of a longer clamper 12 to the piston rod 11 can be further facilitated.