A suspension sub frame structure, comprising: a suspension sub frame configured to extend substantially in a vehicle longitudinal direction and supporting a suspension member of a front wheel; an extension frame configured to extend substantially forwardly from a front end of the suspension sub frame and having lower rigidity against a load applied substantially in the vehicle longitudinal direction than the suspension sub frame; and a cross member configured to extend substantially in a vehicle width direction on an inward side, in the vehicle width direction, of the suspension sub frame and the extension frame, wherein said cross member is joined to said suspension sub frame and said extension frame at a joint area which is formed continuous substantially in the vehicle longitudinal direction, characterized in that said cross member comprises an upper-face joint portion which is joined to an upper face of said extension frame and an upper face of said suspension sub frame at said joint area and a lower-face joint portion which is joined to a lower face of said extension frame and a lower face of said suspension sub frame at said joint area, wherein an outer edge portion of a front-side-portion upper member and an outer edge portion of a side-portion upper member of the cross member are respectively joined to an upper face portion of a rear end portion of the extension frame and an upper face portion of a front end portion of the suspension sub frame as the upper-face joint portion, and wherein both end portions (159c, 159c) of a lower member of the cross member are respectively joined to the lower face portion of the rear end portion of the extension frame and the lower face portion of the front end portion of the suspension sub frame as the lower-face joint portion. a suspension sub frame configured to extend substantially in a vehicle longitudinal direction and supporting a suspension member of a front wheel; an extension frame configured to extend substantially forwardly from a front end of the suspension sub frame and having lower rigidity against a load applied substantially in the vehicle longitudinal direction than the suspension sub frame; and a cross member configured to extend substantially in a vehicle width direction on an inward side, in the vehicle width direction, of the suspension sub frame and the extension frame, wherein said cross member is joined to said suspension sub frame and said extension frame at a joint area which is formed continuous substantially in the vehicle longitudinal direction, characterized in that said cross member comprises an upper-face joint portion which is joined to an upper face of said extension frame and an upper face of said suspension sub frame at said joint area and a lower-face joint portion which is joined to a lower face of said extension frame and a lower face of said suspension sub frame at said joint area, wherein an outer edge portion of a front-side-portion upper member and an outer edge portion of a side-portion upper member of the cross member are respectively joined to an upper face portion of a rear end portion of the extension frame and an upper face portion of a front end portion of the suspension sub frame as the upper-face joint portion, and wherein both end portions (159c, 159c) of a lower member of the cross member are respectively joined to the lower face portion of the rear end portion of the extension frame and the lower face portion of the front end portion of the suspension sub frame as the lower-face joint portion.