A frame structure for a scooter-type vehicle, comprising: a header pipe for steerably supporting a handle and a front fork, the handle being grasped by a driver, and the front fork being connected to the handle; a hollow main pipe extending downward from the header pipe toward the rear of the vehicle; a pair of left and right lower frames (30L, 30R) joined to a lower part of the main pipe and both extending from the lower part of the main pipe toward the rear of the vehicle; front-end parts (22L, 22R) of both of the left and right lower frames (30L, 30R) being joined to the lower part of the main pipe; a cross pipe extending in a lateral direction is joined to both rear-end parts (31L, 31R) of the left and right lower frames (30L, 30R); and front-end parts (71L, 71R) of a pair of left and right rear frames (70L, 70R) extending upward and to the rear of the vehicle being joined to respective left and right end parts (63L, 63R) of the cross pipe, characterized in that the main pipe has an expanding tube part expanding in a lateral direction at a bottom end, and each of the left and right lower frames (30L, 30R) comprises: a front-end joining part (32L, 32R) joined to a left or a right lateral surface (21L, 21R) of the expanding tube part a rear-end joining part (33L, 33R) joined to a left or a right end circumference (61L, 61R) of the cross pipe; and an intermediate part (34L, 34R) provided to an area between the front-end joining part (32L, 32R) and the rear-end joining part (33L, 33R), wherein a height of the intermediate part (34L, 34R) is smaller than a height of the rear-end joining part (33L, 33R). a header pipe for steerably supporting a handle and a front fork, the handle being grasped by a driver, and the front fork being connected to the handle; a hollow main pipe extending downward from the header pipe toward the rear of the vehicle; a pair of left and right lower frames (30L, 30R) joined to a lower part of the main pipe and both extending from the lower part of the main pipe toward the rear of the vehicle; front-end parts (22L, 22R) of both of the left and right lower frames (30L, 30R) being joined to the lower part of the main pipe; a cross pipe extending in a lateral direction is joined to both rear-end parts (31L, 31R) of the left and right lower frames (30L, 30R); and front-end parts (71L, 71R) of a pair of left and right rear frames (70L, 70R) extending upward and to the rear of the vehicle being joined to respective left and right end parts (63L, 63R) of the cross pipe, characterized in that the main pipe has an expanding tube part expanding in a lateral direction at a bottom end, and each of the left and right lower frames (30L, 30R) comprises: a front-end joining part (32L, 32R) joined to a left or a right lateral surface (21L, 21R) of the expanding tube part a rear-end joining part (33L, 33R) joined to a left or a right end circumference (61L, 61R) of the cross pipe; and an intermediate part (34L, 34R) provided to an area between the front-end joining part (32L, 32R) and the rear-end joining part (33L, 33R), a front-end joining part (32L, 32R) joined to a left or a right lateral surface (21L, 21R) of the expanding tube part a rear-end joining part (33L, 33R) joined to a left or a right end circumference (61L, 61R) of the cross pipe; and an intermediate part (34L, 34R) provided to an area between the front-end joining part (32L, 32R) and the rear-end joining part (33L, 33R), wherein a height of the intermediate part (34L, 34R) is smaller than a height of the rear-end joining part (33L, 33R).