A frame structure for a vehicle comprising: a pair of main side rails which extends in the vehicle longitudinal direction and faces each other; and an extension side rail which is connected to a front portion of the main side rail in the vehicle longitudinal direction and extends in the vehicle longitudinal direction, wherein the extension side rail is formed so as to have a substantially C-shaped cross-section, and is formed so that a front portion is lower than a rear portion in the vehicle height direction, the main side rail includes: a main side rail side plate portion which extends in the vehicle height direction: a main side rail upper plate portion which extends from the upper edge of the side plate portion inward in the vehicle width direction; and a main side rail lower plate portion which extends from the lower edge of the side plate portion inward in the vehicle width direction, the extension side rail includes: an extension side rail side plate portion which extends in the vehicle height direction; an extension side rail upper plate portion which extends from the upper edge of the extension side rail side plate portion inward in the vehicle width direction; and an extension side rail lower plate portion which extends from the lower edge of the extension side rail side plate portion inward in the vehicle width direction, characterized in that: the extension side rail side plate portion and the main side rail side plate portion are rigidly connected to each other, and the extension side rail lower plate portion and the main side rail lower plate portion are rigidly connected to each other, and the extension side rail upper plate portion and the main side rail upper plate portion are not rigidly connected to each other. a pair of main side rails which extends in the vehicle longitudinal direction and faces each other; and an extension side rail which is connected to a front portion of the main side rail in the vehicle longitudinal direction and extends in the vehicle longitudinal direction, wherein the extension side rail is formed so as to have a substantially C-shaped cross-section, and is formed so that a front portion is lower than a rear portion in the vehicle height direction, the main side rail includes: a main side rail side plate portion which extends in the vehicle height direction: a main side rail upper plate portion which extends from the upper edge of the side plate portion inward in the vehicle width direction; and a main side rail lower plate portion which extends from the lower edge of the side plate portion inward in the vehicle width direction, a main side rail upper plate portion which extends from the upper edge of the side plate portion inward in the vehicle width direction; and a main side rail lower plate portion which extends from the lower edge of the side plate portion inward in the vehicle width direction, the extension side rail includes: an extension side rail side plate portion which extends in the vehicle height direction; an extension side rail upper plate portion which extends from the upper edge of the extension side rail side plate portion inward in the vehicle width direction; and an extension side rail lower plate portion which extends from the lower edge of the extension side rail side plate portion inward in the vehicle width direction, an extension side rail side plate portion which extends in the vehicle height direction; an extension side rail upper plate portion which extends from the upper edge of the extension side rail side plate portion inward in the vehicle width direction; and an extension side rail lower plate portion which extends from the lower edge of the extension side rail side plate portion inward in the vehicle width direction, characterized in that: the extension side rail side plate portion and the main side rail side plate portion are rigidly connected to each other, and the extension side rail lower plate portion and the main side rail lower plate portion are rigidly connected to each other, and the extension side rail upper plate portion and the main side rail upper plate portion are not rigidly connected to each other.