A bolting structure of a sub-frame includes a reinforcer mounting the sub-frame through a side member. The reinforcer includes: a reinforcing member disposed to intersect with the side member in a cross shape and bolted, together with the side member, to a lower panel; an inner pipe disposed at the side member from surfaces interfacing between the reinforcing member and the side member; an outer pipe disposed at the reinforcing member from the surfaces interfacing between the reinforcing member and the side member and inserted into the sub-frame; and a bolt bolted to the inner pipe through the outer pipe and fixing the sub-frame.