A rotary structural body includes a rotation body that rotates in cooperation with a rotation shaft. The rotation body includes an engaging portion engaged with an engaged portion arranged on one of the rotation shaft and a transmission member that transmits rotation of the rotation shaft to the rotation body. The rotary structural body further includes a detector that detects a rotation angle of the rotation shaft in accordance with rotation of the rotation body and a support arranged on the engaged portion to extend across a circumferential gap between the engaged portion and the engaging portion.