Power tool

A power tool comprising:

• a tool bit;
• a brushless motor having a rotor, wherein the motor drives the tool bit by rotation of the rotor;
• a battery detachably coupled to the power tool, wherein the battery provides direct current to the brushless motor;
• and
• a control device to operate the brushless motor via the battery, wherein the control device includes an advance angle controlling section to control an advance angle of the brushless motor based upon indexes that reflect a working condition of the tool bit when the brushless motor operates, the advance angle indicating phase differences between an induced voltage and a winding current, thereby improving the output efficiency of the power tool based upon said indexes in relation to voltage and current of the battery during operation of the brushless motor, wherein the control device operates the brushless motor so as to decrease a difference between the measured torque in hard joint operation in which the tool bit rotates by first angle until a tightening operation by the tool bit is completed and the measured torque in soft joint operation in which the tool bit rotates by second angle which is smaller than the first angle until a tightening operation by the tool bit is completed.