Second ground plane (18), substantially rectangular in shape, is connected
to first ground plane (17) mounted with a data processing circuit, and radio
communication circuit (12) makes radio communications at communication wavelength
. A portion of second ground plane (18) from ground connecting means
(14) to ground connecting means (33) along continuous edges of second
ground plane (18) acts as an antenna. However, since the longest distance
a between a plurality of ground connecting means (14, 33) along the continuous
edges of second ground plane (18) satisfies "a/2" (where
is a coefficient equal to or larger than "1"), its resonant frequency is
sufficiently higher than the communication frequency of the radio communication circuit.