A rotary structural body, comprising: a drive gear configured to rotate in cooperation with a steering shaft and including an engaging portion engaged with an engaged portion, the engaged portion being arranged on one of the steering shaft and a transmission member configured to transmit rotation of the steering shaft to the drive gear; a detector (51,52) configured to detect a rotation angle of the steering shaft in accordance with rotation of the drive gear; the rotary structural body being characterized by further comprising: a support (40:40a;40b) arranged on the engaged portion and extending across a circumferential gap between the engaged portion and the engaging portion in a state in which the engaged portion and the engaging portion are fitted to each other; wherein the support (40:40a;40b) includes: a fixed end (41a;41b) located at an upstream side in a coupling direction in which the drive gear is coupled to one of the steering shaft and the transmission member, a contact portion (42a;42b) located at a downstream side in the coupling direction in contact with the engaging portion of the drive gear, and a guide portion (43a;43b) extending from the fixed end (41a;41b) to the contact portion (42a;42b) and separated from the engaging portion by a distance that decreases at positions closer to the contact portion (42a;42b); wherein the engaged portion includes a guide groove, the engaging portion includes a guide projection, the support (40:40a;40b) includes a spring piece (40:40a;40b) arranged on at least one of opposing walls of the groove opposed to the guide projection, and the spring piece (40:40a;40b) extends across a circumferential gap between the at least one of the opposing walls of the guide groove and the guide projection. a drive gear configured to rotate in cooperation with a steering shaft and including an engaging portion engaged with an engaged portion, the engaged portion being arranged on one of the steering shaft and a transmission member configured to transmit rotation of the steering shaft to the drive gear; a detector (51,52) configured to detect a rotation angle of the steering shaft in accordance with rotation of the drive gear; the rotary structural body being characterized by further comprising: a support (40:40a;40b) arranged on the engaged portion and extending across a circumferential gap between the engaged portion and the engaging portion in a state in which the engaged portion and the engaging portion are fitted to each other; wherein the support (40:40a;40b) includes: a fixed end (41a;41b) located at an upstream side in a coupling direction in which the drive gear is coupled to one of the steering shaft and the transmission member, a contact portion (42a;42b) located at a downstream side in the coupling direction in contact with the engaging portion of the drive gear, and a guide portion (43a;43b) extending from the fixed end (41a;41b) to the contact portion (42a;42b) and separated from the engaging portion by a distance that decreases at positions closer to the contact portion (42a;42b); wherein the engaged portion includes a guide groove, the engaging portion includes a guide projection, the support (40:40a;40b) includes a spring piece (40:40a;40b) arranged on at least one of opposing walls of the groove opposed to the guide projection, and the spring piece (40:40a;40b) extends across a circumferential gap between the at least one of the opposing walls of the guide groove and the guide projection. a support (40:40a;40b) arranged on the engaged portion and extending across a circumferential gap between the engaged portion and the engaging portion in a state in which the engaged portion and the engaging portion are fitted to each other; wherein the support (40:40a;40b) includes: a fixed end (41a;41b) located at an upstream side in a coupling direction in which the drive gear is coupled to one of the steering shaft and the transmission member, a contact portion (42a;42b) located at a downstream side in the coupling direction in contact with the engaging portion of the drive gear, and a guide portion (43a;43b) extending from the fixed end (41a;41b) to the contact portion (42a;42b) and separated from the engaging portion by a distance that decreases at positions closer to the contact portion (42a;42b); wherein the engaged portion includes a guide groove, the engaging portion includes a guide projection, the support (40:40a;40b) includes a spring piece (40:40a;40b) arranged on at least one of opposing walls of the groove opposed to the guide projection, and the spring piece (40:40a;40b) extends across a circumferential gap between the at least one of the opposing walls of the guide groove and the guide projection.