Disclosed is an automatic correction apparatus for a trajectory of a projectile from a firearm of which a sight and a gun barrel are installed in parallel, the apparatus including: a distance measurer which is installed in parallel with the sight and measures distance from a target to be hit; a central processing unit which calculates a correction value for parallelization between the gun barrel and the sight so that a trajectory curve and the target can intersect with each other on the basis of the distance measured by the distance measurer; and a parallelization adjuster which connects the sight and the gun barrel and adjusts axial parallelization between a sight line of the sight and the gun barrel on the basis of the parallelization correction value calculated by the central processing unit. With this, the trajectory of the projectile is automatically corrected in accordance with the distance from the target, and thus quick and correct aiming and firing are possible.