A vehicle body frame structure of a straddle-type vehicle, the vehicle body frame structure comprising: a main frame; and a rear frame connected to the main frame by a fastener member and extending rearward from the main frame, wherein the main frame includes a first member and a second member which are connected to each other, the main frame includes a first connection portion and a second connection portion which are connected to the rear frame, the first connection portion is provided at the first member, the second connection portion is provided at the second member, and the first connection portion and the second connection portion are spaced apart from each other in a direction which is perpendicular to a vehicle width direction of the straddle-type vehicle, wherein the rear frame includes a first connected portion connected in a first restricting direction to the first connection portion by a first fastener member, and a second connected portion connected in a second restricting direction to the second connection portion by a second fastener member, wherein the first restricting direction is orthogonal to the second restricting direction characterized in that the vehicle body frame structure further comprises a support member connected to the main frame by a fastener member, wherein the fastener member is the first fastener member or the second fastener member, and wherein one of the first connected portion and the second connected portion is connected to corresponding one of the first connection portion and the second connection portion, by the fastener member, together with the support member, wherein the main frame includes a pair of main frames placed in the vehicle width direction, and the fastener member used to connect the support member to the main frame includes a pair of fastener members placed in the vehicle width direction, wherein the support member is placed between the pair of main frames, the pair of fastener members are inserted into the pair of main frames, respectively, in a state in which the pair of fastener members face each other in the vehicle width direction, from outside of the pair of main frames in the vehicle width direction, the support member includes a pair of fastened portions placed in the vehicle width direction, the pair of fastened portions being engageable with the pair of fastener members, respectively, and wherein the support member is connected to each of the pair of main frames in such a manner that the pair of main frames are pressed against the support member and the pair of fastener members are engaged with the pair of fastened portions, respectively, and wherein one of the first connected portion and the second connected portion is placed between the support member and the main frame. a main frame; and a rear frame connected to the main frame by a fastener member and extending rearward from the main frame, wherein the main frame includes a first member and a second member which are connected to each other, the main frame includes a first connection portion and a second connection portion which are connected to the rear frame, the first connection portion is provided at the first member, the second connection portion is provided at the second member, and the first connection portion and the second connection portion are spaced apart from each other in a direction which is perpendicular to a vehicle width direction of the straddle-type vehicle, wherein the rear frame includes a first connected portion connected in a first restricting direction to the first connection portion by a first fastener member, and a second connected portion connected in a second restricting direction to the second connection portion by a second fastener member, wherein the first restricting direction is orthogonal to the second restricting direction characterized in that the vehicle body frame structure further comprises a support member connected to the main frame by a fastener member, wherein the fastener member is the first fastener member or the second fastener member, and wherein one of the first connected portion and the second connected portion is connected to corresponding one of the first connection portion and the second connection portion, by the fastener member, together with the support member, wherein the main frame includes a pair of main frames placed in the vehicle width direction, and the fastener member used to connect the support member to the main frame includes a pair of fastener members placed in the vehicle width direction, wherein the support member is placed between the pair of main frames, the pair of fastener members are inserted into the pair of main frames, respectively, in a state in which the pair of fastener members face each other in the vehicle width direction, from outside of the pair of main frames in the vehicle width direction, the support member includes a pair of fastened portions placed in the vehicle width direction, the pair of fastened portions being engageable with the pair of fastener members, respectively, and wherein the support member is connected to each of the pair of main frames in such a manner that the pair of main frames are pressed against the support member and the pair of fastener members are engaged with the pair of fastened portions, respectively, and wherein one of the first connected portion and the second connected portion is placed between the support member and the main frame.