A computer controlled electric syringe comprising: a piston unit comprising: a piston comprising a first end, a second end and a hollow portion communicating with an opening formed at the first end, the piston being configured to press and move a rubber plug (251, 261) of a cartridge (250, 260), in which a medicinal solution is enclosed, by the second end to discharge the medicinal solution from the cartridge (250, 260) through an injection needle; a ball screw comprising a screw shaft and a nut portion that is screwed into the screw shaft, the screw shaft being loosely inserted into the hollow portion of the piston through the opening, the nut portion being coupled to the piston; a screw shaft supporting portion rotatably supporting the screw shaft of the ball screw; a rotation preventing portion configured to prevent the nut portion of the ball screw from being rotated; a piston supporting portion supporting the piston to be movable in an axial direction of the screw shaft along with the nut portion by the rotation of the screw shaft; a drive motor; a drive mechanism configured to transmit a rotative force output from the drive motor to the screw shaft of the ball screw of the piston unit; a motor controller configured to control the drive motor; a stroke sensor configured to detect a position of the piston of the piston unit; a power supply unit configured to supply power to the drive motor, the motor controller, and the stroke sensor; a main processor connected to the motor controller, the stroke sensor, and the power supply unit; a main case covering the piston unit, the drive mechanism, the drive motor, the motor controller, the power supply unit, the stroke sensor, and the main processor; and a cartridge holder socket connected to the main case and configured to allow the piston to pass through while maintaining water-tightness, the cartridge holder socket being configured to receive a cartridge holder into which the cartridge is inserted; wherein the screw shaft supporting portion rotatably supports only one end of the screw shaft of the ball screw so as to omit to support the other end of the screw shaft; characterized by a top base and a bottom base configured to fix the screw shaft supporting portion and the piston supporting portion while sandwiching the screw shaft supporting portion and the piston supporting portion from above and below by using a reinforcing ring, the reinforcing ring being fitted into a groove portion which is formed when the bottom base and top base are assembled together and firmly fixes the bottom base and the top base; and in that the screw shaft supporting portion includes a thrust bearing and two radial bearings and rotatably supports the screw shaft. a piston comprising a first end, a second end and a hollow portion communicating with an opening formed at the first end, the piston being configured to press and move a rubber plug (251, 261) of a cartridge (250, 260), in which a medicinal solution is enclosed, by the second end to discharge the medicinal solution from the cartridge (250, 260) through an injection needle; a ball screw comprising a screw shaft and a nut portion that is screwed into the screw shaft, the screw shaft being loosely inserted into the hollow portion of the piston through the opening, the nut portion being coupled to the piston; a screw shaft supporting portion rotatably supporting the screw shaft of the ball screw; a rotation preventing portion configured to prevent the nut portion of the ball screw from being rotated; a piston supporting portion supporting the piston to be movable in an axial direction of the screw shaft along with the nut portion by the rotation of the screw shaft; a drive motor; a drive mechanism configured to transmit a rotative force output from the drive motor to the screw shaft of the ball screw of the piston unit; a motor controller configured to control the drive motor; a stroke sensor configured to detect a position of the piston of the piston unit; a power supply unit configured to supply power to the drive motor, the motor controller, and the stroke sensor; a main processor connected to the motor controller, the stroke sensor, and the power supply unit; a main case covering the piston unit, the drive mechanism, the drive motor, the motor controller, the power supply unit, the stroke sensor, and the main processor; and a cartridge holder socket connected to the main case and configured to allow the piston to pass through while maintaining water-tightness, the cartridge holder socket being configured to receive a cartridge holder into which the cartridge is inserted; wherein the screw shaft supporting portion rotatably supports only one end of the screw shaft of the ball screw so as to omit to support the other end of the screw shaft; characterized by a top base and a bottom base configured to fix the screw shaft supporting portion and the piston supporting portion while sandwiching the screw shaft supporting portion and the piston supporting portion from above and below by using a reinforcing ring, the reinforcing ring being fitted into a groove portion which is formed when the bottom base and top base are assembled together and firmly fixes the bottom base and the top base; and in that the screw shaft supporting portion includes a thrust bearing and two radial bearings and rotatably supports the screw shaft.