the flag set in the output register 51 and analyzes the contents then through the serial interface 50 the host apparatus requests the optical transmission control circuit 1 to send the digital value corresponding to the flag indicating an anomaly according to the flag contents accordingly the computation circuit 21 extracts the digital value requested by the host apparatus from the maximum value minimum value storage section 12 and outputs the digital value to the host apparatus through the serial interface 50 accordingly the optical transmission control circuit 1 stores and updates the digital value of the a d conversion result register 32 in the maximum value minimum value storage section 12 accordingly after detecting the digital value crossing the threshold value and outputting an error notification to the host apparatus if the optical transmission control circuit 1 acquires the digital value crossing the digital value it updates the maximum value and or the minimum value so that it is made possible for the host apparatus to read the digital value based on the updated maximum value and or the updated minimum value and the more accurate digital value can be provided for the host apparatus the optical transmission control circuit 1 generates the flag value requested by the host apparatus using the general purpose register 22 and outputs the flag value to the host apparatus through the output register 51 so that the flag value need not be stored in the optical transmission control circuit 1 and the storage capacity can be reduced further the optical transmission control circuit 1 starts flag generation after inputting the error check request from the host apparatus and thus the contents of the maximum value minimum value storage section 12 in the most recent state.