The identifier 103 stores the three light patterns of red, green, and blue in the column of “Light Pattern” for the light receiver R4 shown in FIG. 9A, for example according to the light intensity signals from the photodetectors of red (R), green (G), and blue (B) in the light receiver R4. The identifier 103 also stores “Not Exceeding Threshold” in the column of “Reception Level” shown in FIG. 9A, because the signal reception levels of the three light intensity signals are not higher than the predetermined threshold. Then the identifier 103 decides that the light receiver R4 is unable to receive the signal, instead of performing the matching between the light pattern “Red: Off, Green: Off, Blue: Off” stored in the column of “Light Pattern” for the light receiver R4 shown in FIG. 9A, and the light pattern “Red: On, Green: Off, Blue: On” stored in the storage device 13 in the column of “Allocated Color”, and stores “N/A” indicating that the color is unidentified, in the column of “Received Color” for the light receiver R4. The decider 104 stores “Wrong” in the column of “Decision Result” for the light receiver R4 shown in FIG. 9A, because the identifier 103 has decided that the light receiver R4 is unable to receive the signal.
