PURPOSE: To provide a self-diagnosing device which can perform automatically diagnosis in relation to a rotary driving mechanism.

CONSTITUTION: In a web offset printing press, self-diagnosis in relation to a rotary driving mechanism having a motor 10 is performed. A rotary command device 20 gives a command value I to a motor 10 so that the motor turns at a fixed rotary speed. Real rotary condition of the motor 10 is measured as a measured value M by a rotary encoder 30. A rotary speed comparison device 40 performs information of abnormality in the case where a difference between a rotary speed V to be obtained through the measured value M and the command value I is at least within an allowable range. Then an angular velocity change detecting device 50 performs information of abnormality in the case where a change of an angular velocity ω to be obtained through the measured value M is at least within the allowable range.