The present invention is directed to an assembly structure designed to improve the process of accurately assembling an instrument panel supported by a steering support member in a vehicle. The assembly structure includes side brackets (16), (17) arranged to mount both ends of a steering support member (15) to a car body. Side bracket (16) includes an extended portion (20) provided with an instrument panel mounting seat (22), a car body mounting seat (23), and a side trim mounting seat (24). The upper side of an instrument panel (14) is mounted to the cylindrical portion of the steering support member (15) and the lower end portion thereof is mounted to the instrument panel mounting seat (22). The car body mounting seat (23) of the side bracket (16) is mounted to the car body, while a dash side trim (31) is mounted to the side trim mounting seat (24). Instead of mounting the instrument panel (14) so as to be directly coupled to the car body, it is mounted to the car body via the steering support member (15). Since the processes of assembling the instrument panel (14) and the body panel are separated from each other, the instrument panel can be assembled accurately with an improved appearance.