This invention provides an antenna module comprising: a helical antenna 1 including a base 2 and a pair of terminals 4, 5 and a helical area formed on the base 2; a power supply 7 for supplying power to one of the pair of terminals 4, 5 of the helical antenna 1; an opening connected to the other of the pair of terminals; an antenna substrate 9 on which the antenna 7 is mounted; a grounding area 10 formed in the vicinity of the power supply 7; and a peripheral conductor 16 formed at least a portion on the periphery of the antenna substrate 9, wherein the peripheral length of the peripheral conductor 16 formed on the periphery of the antenna substrate 9 is nearly integer times as long as ¼ wavelength of a resonance frequency.