A joint structure includes a sandwiched structural body comprising a core layer having opposite surfaces and a pair of first and second outer skin layers covering the opposite surfaces, respectively, and a substantially concave joint region defined in the first outer skin layer and comprising a side wall and a bottom wall contiguous to the side wall, the bottom wall being joined to the second outer skin layer. The side wall of the joint region is of a tapered shape. The sandwiched structural body is formed primarily of synthetic resins, and may additionally include reinforcing fibers.