Disclosed is a tape cutting device of an automatic roll tape cutter capable of automatically cutting a tape fed out by a proper length. The tape cutting device includes: a frame detachably assembled onto the front surface of a case of the tape cutter, and having a slot formed therethrough and movable blade guide channels formed on both sides of the rear surface thereof; a fixed blade mounted on the upper portion of the rear surface of the frame; a movable blade mounted on the lower portion of the rear surface of the frame in such a manner as to be parallel with the fixed blade, and having a horizontal hole formed on one side of the lower end thereof, an inclined hole formed on the other side of the lower end thereof, and a sensor inserting hole formed at the center thereof; and a lubricating oil supply part for supplying lubricating oil to sides of the blades or a side of the fixed blade or the movable blade.