To provide an original film proof reading device in digital printing with high efficiency and capable of exactly conducting proof reading of original film.

In output process of the digital printing, raster type data obtained by raster conversion of page description data is stored in an exclusive disc (a first proofreading). And, the latest page description data corrected as the result of the proofreading is converted into the raster type data and stored in the exclusive disc (a second proofreading). And, when the original film proof reading is performed, the data of the first proofreading and the data of the second proofreading are read out. In this case, the data expressed in dots is converted into the data expressed in a value of density by the pixel if necessary. And comparison of the first proofreading with the second proofreading is performed by the pixel based on an allowable value. And, a part corrected by the proofreading is specified by extracting the pixel whose difference of density by every pixel excesses the allowable value.