The present invention relates to a metal nut insert device of a plastic molded product for automobile engine parts, in a metal nut insert device using a robot, the insert configured to operate a piston rod with pneumatic pressure while being mounted on the tip of the robot. A cylinder; An upper fixing flange and a lower adjustment flange mounted on the front end side of the insert cylinder so that the piston rod of the insert cylinder passes through; A centering master mounted between the upper fixing flange and the lower adjustment flange and configured to adjust the lower adjustment flange to be inclined at a fine inclination angle in all directions based on the horizontal; A clamp housing fixedly attached to the front end of the lower adjustment flange and configured to insert a metal nut in accordance with an inclination angle of an upper surface of a nut insertion hole formed in a plastic molded product by an operation of adjusting an inclination angle like a lower adjustment flange; An elevating operation rod mounted to enable elevating movement along a moving passage formed inside the clamp housing according to the protruding and disengaging operation of the piston rod of the insert cylinder; It characterized in that it is configured to include a nut tongs configured to grip the inner diameter of the metal nut while being mounted on the tip of the clamp housing, and a plurality of gripping portions divided by the tip of the lifting and lowering rod are elastically spread outward. It is an invention.