An installation structure of a rotation detector for detecting a rotation amount of a rotational shaft such as a kingpin. The installation structure prevents wear and damage to an input shaft of the rotation detector caused by a radial force applied to the input shaft and has high detection accuracy by eliminating slack between the rotational shaft and the input shaft. The rotation detector is fixed to a support member rotatably supporting the rotational shaft such that the input shaft is substantially coaxial with the rotational shaft. The input shaft is coupled to the rotational shaft through a rotation transmitting means. The rotation transmitting means permits the axis of the input shaft to be offset from the axis of the rotational shaft and limits relative rotation between the input shaft and the rotational shaft. When the axis of the input shaft is offset from the axis of the rotational shaft, the radial force applied to the input shaft from the rotational shaft is absorbed and mitigated, and the rotation of the rotational shaft is transmitted to the input shaft without loss.