A method and diagnosis apparatus are provided. A velocity measuring section measures a bit length of communication carried out between nodes based upon a communication signal inputted from a communication bus. The measured bit length is input to a control section, which sets a bit length based upon input from the velocity measuring section and arranges the communication section to decode the communication signal. The communication section specifies a frame reception time and a frame ID as an identification code of a time slot in which a frame is transmitted. Each time the frame is received after setting the bit length, the control section records information of the frame reception time and the frame ID inputted from the communication section in a log file. The control section determines whether the frame is transmitted in a constant period from each time slot based upon the content of the log file.