An impact rotary tool comprising: a drive source supplied with power from a battery pack (V) to rotate a hammer with a drive shaft ; an output shaft rotated when struck by the hammer ; a strike detector that detects striking by the hammer ; a rotation speed detector that detects a rotation speed of the drive shaft ; a rotation angle detector that detects a rotation angle of the output shaft during a strike interval from when the strike detector detects a preceding strike to when the strike detector detects a following strike; a torque calculator that calculates striking energy from an input rotation average speed during the strike interval, which is calculated based on the rotation speed of the drive shaft detected by the rotation speed detector , and calculates a tightening torque (T) based on the calculated striking energy and the rotation angle of the output shaft during a strike interval detected by the rotation angle detector ; and a controller that controls the drive source based on the tightening torque (T) calculated by the torque calculator , characterized in that the controller performs PWM control to limit strike force applied when voltage of the battery pack (V) is high and controls the drive source to maintain strike force even when the voltage of the battery pack (V) is low, and in that the impact rotary tool also comprises a current detector that detects current supplied to the drive source , wherein the controller decreases a target rotation speed of the drive source when the current detected by the current detector is greater than or equal to an impact determination threshold (TI). a drive source supplied with power from a battery pack (V) to rotate a hammer with a drive shaft ; an output shaft rotated when struck by the hammer ; a strike detector that detects striking by the hammer ; a rotation speed detector that detects a rotation speed of the drive shaft ; a rotation angle detector that detects a rotation angle of the output shaft during a strike interval from when the strike detector detects a preceding strike to when the strike detector detects a following strike; a torque calculator that calculates striking energy from an input rotation average speed during the strike interval, which is calculated based on the rotation speed of the drive shaft detected by the rotation speed detector , and calculates a tightening torque (T) based on the calculated striking energy and the rotation angle of the output shaft during a strike interval detected by the rotation angle detector ; and a controller that controls the drive source based on the tightening torque (T) calculated by the torque calculator , characterized in that the controller performs PWM control to limit strike force applied when voltage of the battery pack (V) is high and controls the drive source to maintain strike force even when the voltage of the battery pack (V) is low, and in that the impact rotary tool also comprises a current detector that detects current supplied to the drive source , wherein the controller decreases a target rotation speed of the drive source when the current detected by the current detector is greater than or equal to an impact determination threshold (TI). <EOS>