Abstract: A first aspect of the present invention is carbon fiber wherein the surface of a monofilament has a center line average roughness Ra of 6.0 nm or more and 13 nm or less, and the monofilament has a long diameter/short diameter ratio of 1.11 or more and 1.245 or less. A second aspect of the present invention is carbon fiber precursor acrylic fiber wherein the surface of a monofilament has a center line average roughness Ra of 18 nm or more and 27 nm or less, and the monofilament has a long diameter/short diameter ratio of 1.11 or more and 1.245 or less. The carbon fiber according to the first aspect is obtained by stabilizing and carbonizing under specific conditions the carbon fiber precursor acrylic fiber according to the second aspect.

Abstract: A tip end of a pivot shaft is hollowed out and a threaded section is formed inside. A pivot support shaft is attached to left and right pivot support sections by screwing a bolt into the threaded section. Since the only parts of the pivot shaft and the bolt section that protrude from the left and right pivot support sections are the respective head sections, it is possible to make the length of protrusion from the left and right pivot support sections constant. As a result, all that protrudes from the left pivot support section arranged on the bolt side or the right pivot support section is the bolt head. Furthermore, it is possible to make the depth of the left pivot support section arranged on the bolt side or the depth of the right pivot support section shallower.