In recent years, the industrial researchers have paid attention to the not only position control but also force control. The one target of force control researches is the injection molding machine. The conventional force control system uses the force sensor to detect the inserted force. However, the force control system using force sensor has some problems such as the noise, the frequency band and so on. This paper newly proposes that the reaction torque observer is applied to the injection molding machine using ball screw. The reaction torque observer solves this problem of force sensor. However, it is well-known that the ball screw system has the resonant frequency caused by the torsion phenomenon. Hence, this torsion vibration affects both performances of force control and reaction torque estimation. This paper proposes a new reaction torque observer considering the torsion phenomenon and the friction torque. The proposed reaction torque observer estimates the reaction torque accurately. Moreover, as the outside of proposed reaction torque observer has its friction model, this friction model can be tuned without considering the stability condition of force control feedback system. The validity of proposed force control system is conflrmed by the experimental results. This paper realizes the low cost and space-saving injection molding machine by using the proposed force sensor-less control method.