An apparatus is used in the production of a monocrystalline rod by Czochralski method and is adapted for measuring the diameter of the monocrystalline rod at the melt surface of the melt by processing image signal derived from an image sensor. The measurement of the diameter is conducted by determining the diameter of the monocrystalline rod at the melt surface by processing the output signal from the image sensor, and multiplying the rod image diameter with a function of the difference between the initial melt surface level and a melt surface corrected in accordance with a change in the temperature in the apparatus chamber or a change in the leval of the electrical power supplied to a heater for heating a crucible.