PURPOSE: To improve working efficiency upon investigating the cause of deterioration of a picture by a method wherein the cause of faulty picture or the content of successively effected test mode is determined by a control means in accordance with the input of an inputting means while the indication of the cause of faulty picture or the indication of the content of successively effected test mode is effected by an indicating means.

CONSTITUTION: A picture forming device is equipped with an image forming means 3, forming an image on a sheet, a mode setting means 13, setting a test mode for testing the cause of faulty picture, a control means 11, forming a plurality of sample pictures by changing the operating conditions of the image forming means when a test mode is set by the means 13, an inputting means 13, inputting manually the result of comparison of a plurality of sample pictures, and an indicating means 14. The control means 11 determines the cause of flauty picture or the content of successively effected test mode in accordance with the input of the inputting means 13 and indicates the indication of the cause of faulty picture or the indication with respect to the content of the successively effected test mode through the indicating means 14.