a al 2 o 3 film 13 having been damaged by the etching only its exposed portion may be removed by wet etching so that a smooth and flat sio 2 film 14 is exposed subsequently as shown in fig2 a chromium film 15 damaged by the etching may be removed by wet etching so that a smooth and flat sio 2 film 14 is exposed by doing so a four level diffractive optical element of rectangular shape having uniform level difference and having a side wall facing the step extending perpendicularly to the bottom surface can be produced in this embodiment while not shown in the drawings an al reflection film may be formed on the diffraction grating surface of a four level diffractive optical element thus produced by using a vacuum deposition apparatus a reflection type diffractive optical element can be manufactured thereby as a fourth embodiment without forming the al 2 o 3 film 13 a which is an auxiliary film on the substrate 12 in the third embodiment the sio 2 film 14 main film may be directly formed on the substrate 12 in that occasion the number of accumulated layers for the sio 2 film may be increased from three layers to seven layers and the thicknesses of the sio 2 films 4 may be set to 263 nm 233 nm 239 nm 243 nm 244 nm 246 nm and 246 nm respectively in an order from the substrate 12 side also the thicknesses of al 2 o 3 films 13 formed on the sio 2 film 14 may be set to 30 nm 24 nm 20 nm 19 nm 17 nm and 17 nm respectively in an order from the bottom sio 2.