PURPOSE: To automatically check the abnormal open circuit of an antenna line and the abnormal short circuit to the earth.

CONSTITUTION: An antenna element 1 is electrically earthed when it is stored. A detector means 110 detects an open state even when the element 1 is stored. Otherwise a diagnostic means 120 decides an open fault caused by the discontinuity or the defective connection of an antenna line 100 or a short fault due to the connection secured to the earth when a short fault is detected when the element 1 is extended. These detecting results are shown on a display means 13.