To provide the rolled film diameter measuring instrument for a packing machine which can automatically measure the diameter of a rolled film.

A 4th driving motor 4 is linked to the rolled film 15, which is rotated in a specific direction to feed a beltlike film 16 out and guide it to a bag maker 18. A rotational angle measuring instrument 51 which measures the angle of rotation of the rolled film 15 is fitted to the rotary shaft of the rolled film and a moving distance measuring instrument 53 is fitted which comes into contact with the peripheral surface of the rolled film 15 and detects the movement distance of the beltlike film 16 wound around the rolled film before being drawn to extend. The 4th driving motor 40 is rotated for a certain time and the angle of rotation and the moving distance at this time are measured by the respective measuring instruments and sent to an arithmetic processor, which finds the rolled film diameter. The found diameter is sent to a speed controller 38 and used to initially set the rotating speed of the 4th driving motor 40 at its start.