PURPOSE: To discriminate a line speed without using a special measuring apparatus by measuring a passage time of a data carrier passing a communicable distance and simultaneously discriminating whether a speed is stable based on a communication amount by which transmission of data to a data carrier is effected and the passage speed thereof.

CONSTITUTION: After an auto command is delivered, waiting for approach of a data carrier 12 is effected, and when the data carrier 12 approaches, a timer for measurement of a time is started. It is checked whether the data carrier 12 passes through a communicable area. When the carrier is parted from the communicable area, a timer for measurement of a time is stopped and a current time is T1. A line speed is computed and processed. A line speed V, a communicable range L, and a passage time T are orderly switched to a numerical display part 26 for display. It is discriminated whether the line speed V is within a range having an enough margin for an upper limit speed V1 responding to the number of bites of an auto command, and a discriminating result is displayed by a status display LED 27.