Abstract: A cap attaching/detaching device 20 is a device for attaching or detaching a cap 141 in which a screw portion is formed to or from a test tube 14 by rotating the cap 141. A cap grip portion 21 grips the cap 141. A rotation mechanism 22 attaches or detaches the cap 141 to or from the test tube 14 by rotating the cap 141 by rotating the cap grip portion 21. The rotation mechanism 22 has an axial portion 221, and a nut portion 222. The axial portion 221 has an outer peripheral surface on which a screw thread 223 is formed. The nut portion 222 has an inner peripheral surface on which a screw groove 224 to be screwed into the screw thread 223 of the axial portion 221 is formed, and rotatably holds the axial portion 221 in a fixed state. A pitch of the screw thread 223 is the same as a pitch of the screw portion formed in the cap 141.