A steering device characterized by comprising: a steering shaft that has one end thereof coupled to a steering member and is extendable and contractable in an axial direction; a column jacket that includes a lower jacket and an upper jacket fitted together to be slidable relative to each other in the axial direction, and by which the steering shaft is rotatably supported; a first bracket that includes a pair of first side plates (20L, 20R) each having a tilt-adjust long groove formed therein, and is fixed on a vehicle body; a second bracket that includes a pair of second side plates (22L, 22R) respectively facing the first side plates and each having a telescopic-adjust long groove formed therein, and is fixed on the upper jacket; and a fastening mechanism that includes: a first fastening member and a second fastening member each having a fastening plate (40, 40P) facing the corresponding first side plate on the opposite side of the corresponding first side plate from the corresponding second side plate, a boss (50, 50P) with a rhombic cross-section that protrudes from a first side plate-side surface (40b, 40Pb) of the fastening plate and is inserted into the tilt-adjust long groove of the corresponding first side plate and the telescopic-adjust long groove of the corresponding second side plate, and a fastening shaft insert hole (42, 42P) passing through the fastening plate and the boss; and a fastening shaft inserted into the fastening shaft insert holes, the fastening mechanism pressing the fastening plates against the corresponding first side plates to fasten the corresponding first side plates to the corresponding second side plates, wherein an outer periphery (50a, 50Pa) of the boss of at least one of the first fastening member and the second fastening member includes a first corner part (61, 61P) and a third corner part (63, 63P) each forming an obtuse angle and facing each other, and a second corner part (62, 62P) and a fourth corner part (64, 64P) each forming an acute angle and facing each other, and the first corner part (61, 61P) is disposed on a steering member side in the axial direction, on a lower side in a tilt direction, while the third corner part (63, 63P) is disposed on a side opposite from the steering member side in the axial direction, on an upper side in the tilt direction, characterized in that a width across the first corner part (61, 61P) and the third corner part (63, 63P) is shorter than a groove width of the telescopic-adjust long groove in a direction orthogonal to the longitudinal direction of the telescopic-adjust long groove. a steering shaft that has one end thereof coupled to a steering member and is extendable and contractable in an axial direction; a column jacket that includes a lower jacket and an upper jacket fitted together to be slidable relative to each other in the axial direction, and by which the steering shaft is rotatably supported; a first bracket that includes a pair of first side plates (20L, 20R) each having a tilt-adjust long groove formed therein, and is fixed on a vehicle body; a second bracket that includes a pair of second side plates (22L, 22R) respectively facing the first side plates and each having a telescopic-adjust long groove formed therein, and is fixed on the upper jacket; and a fastening mechanism that includes: a first fastening member and a second fastening member each having a fastening plate (40, 40P) facing the corresponding first side plate on the opposite side of the corresponding first side plate from the corresponding second side plate, a boss (50, 50P) with a rhombic cross-section that protrudes from a first side plate-side surface (40b, 40Pb) of the fastening plate and is inserted into the tilt-adjust long groove of the corresponding first side plate and the telescopic-adjust long groove of the corresponding second side plate, and a fastening shaft insert hole (42, 42P) passing through the fastening plate and the boss; and a fastening shaft inserted into the fastening shaft insert holes, the fastening mechanism pressing the fastening plates against the corresponding first side plates to fasten the corresponding first side plates to the corresponding second side plates, wherein an outer periphery (50a, 50Pa) of the boss of at least one of the first fastening member and the second fastening member includes a first corner part (61, 61P) and a third corner part (63, 63P) each forming an obtuse angle and facing each other, and a second corner part (62, 62P) and a fourth corner part (64, 64P) each forming an acute angle and facing each other, and the first corner part (61, 61P) is disposed on a steering member side in the axial direction, on a lower side in a tilt direction, while the third corner part (63, 63P) is disposed on a side opposite from the steering member side in the axial direction, on an upper side in the tilt direction, characterized in that a width across the first corner part (61, 61P) and the third corner part (63, 63P) is shorter than a groove width of the telescopic-adjust long groove in a direction orthogonal to the longitudinal direction of the telescopic-adjust long groove.