PURPOSE: To provide a transmission information monitoring device capable of accurately detecting an information error in an interface circuit through which transmission information is passed.

CONSTITUTION: A monitoring pattern generating part 1 selectively extracts at least a used slot information (real line data) from a transmission frame inputted to an interface circuit 3 and an OR gate 2 inserts the extracted information into an unused slot in the pertinent transmission frame. On the output side of the circuit 3, a monitoring pattern detecting part 4 judges coincidence between a monitoring pattern on the same transmission frame and the used slot information which is the original of the monitoring pattern by means of an AND gate 44. Consequently an analog information error generated at random can also be detected in addition to the detection of a digital information error.