To provide an easy-to-assmble retaining structure for retaining a tube that rotatably supports a steering shaft in a housing, and to provide a method of assembling the retaining structure.

A retaining structure 30 retains a lower tube 25 for rotatably supporting a steering shaft, in a relatively slidable housing 12. The retaining structure 30 includes: a pressure member 40 movably provided on the side of the lower tube 25 to press the lower tube 25, in a hole 32 formed in the housing 12; a regulation member 50 fitted to the pressure member 40 to regulate a moving amount in a direction opposite the pressing direction of the pressure member 40, and to regulate the pressure member 40 from coming off from the hole 32 formed in the housing 12; and a biasing member 60 that biases the pressure member 40 toward the lower tube 25.