The present invention provides a narrow band communication vehicle-mounted apparatus capable of performing communication in which there are normally no discrepancies with on-road systems when the apparatus starts up. The narrow band communication vehicle-mounted apparatus comprising: a radiocommunication portion 2 for sending and receiving with an on-road device via an antenna 1, a field intensity measuring portion 3 for detecting a radio field intensity, a control microcomputer 4 for controlling various equipment and a nonvolatile memory 5, wherein the control microcomputer 4 stores in the nonvolatile memory 5 randomly generated communication registration identification data when communication is opened or when the apparatus starts up, and communication is performed using communication registration identification data stored in the nonvolatile memory 5 in a case where the radio field intensity is in a communication range when the apparatus starts up.