A syringe drive unit, comprising: a base plate; a syringe holder which is fixed to a front surface side of the base plate and holds a syringe body; a guide rail which is provided on a rear surface side of the base plate along an axial-line direction of the syringe body mounted to the syringe holder; a slider which is assembled to the guide rail through intermediation of a large number of rolling elements and movable along the guide rail; a plunger pressing member which is fixed to the slider and comprises an arm portion projecting from the rear surface side of the base plate to the front surface side of the base plate, the arm portion holding a plunger housed in the syringe body; a screw shaft which is provided on the rear surface side of the base plate parallel to the guide rail and has predetermined rotation imparted by a motor; and a nut member which is threadedly engaged with the screw shaft to impart a moving amount corresponding to a rotation angle of the motor to the plunger pressing member; characterized in that the guide rail is formed into a substantially rectangular shape in cross-section perpendicular to a moving direction of the slider and installed on the base plate, wherein the slider is formed into a shape of a saddle striding over the guide rail, wherein the base plate is provided with cutout portions on both sides of an installation position of the guide rail, wherein the plunger pressing member comprises a fixed plate fixed to the slider, and wherein the arm portion comprises a pair of arm portions provided upright on the fixed plate and passing through the cutout portions of the base plate. a base plate; a syringe holder which is fixed to a front surface side of the base plate and holds a syringe body; a guide rail which is provided on a rear surface side of the base plate along an axial-line direction of the syringe body mounted to the syringe holder; a slider which is assembled to the guide rail through intermediation of a large number of rolling elements and movable along the guide rail; a plunger pressing member which is fixed to the slider and comprises an arm portion projecting from the rear surface side of the base plate to the front surface side of the base plate, the arm portion holding a plunger housed in the syringe body; a screw shaft which is provided on the rear surface side of the base plate parallel to the guide rail and has predetermined rotation imparted by a motor; and a nut member which is threadedly engaged with the screw shaft to impart a moving amount corresponding to a rotation angle of the motor to the plunger pressing member; characterized in that the guide rail is formed into a substantially rectangular shape in cross-section perpendicular to a moving direction of the slider and installed on the base plate, wherein the slider is formed into a shape of a saddle striding over the guide rail, wherein the base plate is provided with cutout portions on both sides of an installation position of the guide rail, wherein the plunger pressing member comprises a fixed plate fixed to the slider, and wherein the arm portion comprises a pair of arm portions provided upright on the fixed plate and passing through the cutout portions of the base plate.