An electric linear actuator comprising: a housing ; an electric motor mounted on the housing ; a speed reduction mechanism for reducing rotational speed of the electric motor via a motor shaft; and a ball screw mechanism for converting rotational motion of the electric motor transmitted via the speed reduction mechanism to axial linear motion of a driving shaft ; the ball screw mechanism comprising a nut formed with a helical screw groove on its inner circumference and supported by rolling bearings mounted on the housing rotationally but axially immovably, and a screw shaft coaxially integrated with the driving shaft , formed with a helical screw groove on its outer circumference corresponding to the helical screw groove of the nut , inserted into the nut via a large number of balls , and supported axially movably but not rotationally, wherein the housing comprises a first housing on which the electric motor is mounted and a second housing (2b; 2b') to be abutted against an end face of the first housing , wherein at least one (2b; 2b') of the first and second housings is formed with ribs (23; 24) extending from each of securing portions arranged on the periphery of said at least one of the first and second housings and through which fastening bolts are inserted to a containing portion for containing the screw shaft , wherein the securing portions partially project radially outward from the periphery of said at least one of the first and second housings and accommodate the fastening bolts , and wherein the height of each rib (23; 24) arcuately varies from the corresponding securing portion toward the containing portion of said at least one of the first and second housings so that the tangential inclination of the arcuate (23; 24) becomes gradually gentle toward the containing portion , characterized in that the radially outermost end of each rib (23; 24) is set at a position radially outer than the bolt pitch circle diameter (B.C.D) of the fastening bolts . a housing ; an electric motor mounted on the housing ; a speed reduction mechanism for reducing rotational speed of the electric motor via a motor shaft; and a ball screw mechanism for converting rotational motion of the electric motor transmitted via the speed reduction mechanism to axial linear motion of a driving shaft ; the ball screw mechanism comprising a nut formed with a helical screw groove on its inner circumference and supported by rolling bearings mounted on the housing rotationally but axially immovably, and a screw shaft coaxially integrated with the driving shaft , formed with a helical screw groove on its outer circumference corresponding to the helical screw groove of the nut , inserted into the nut via a large number of balls , and supported axially movably but not rotationally, wherein the housing comprises a first housing on which the electric motor is mounted and a second housing (2b; 2b') to be abutted against an end face of the first housing , wherein at least one (2b; 2b') of the first and second housings is formed with ribs (23; 24) extending from each of securing portions arranged on the periphery of said at least one of the first and second housings and through which fastening bolts are inserted to a containing portion for containing the screw shaft , wherein the securing portions partially project radially outward from the periphery of said at least one of the first and second housings and accommodate the fastening bolts , and wherein the height of each rib (23; 24) arcuately varies from the corresponding securing portion toward the containing portion of said at least one of the first and second housings so that the tangential inclination of the arcuate (23; 24) becomes gradually gentle toward the containing portion , characterized in that the radially outermost end of each rib (23; 24) is set at a position radially outer than the bolt pitch circle diameter (B.C.D) of the fastening bolts . <EOS>