PURPOSE: To provide a microstrip patch antenna which can improve the radiation power density of radio waves by supplying an internal patch of an outer slit loading type and an external patch that surrounds the internal patch.

CONSTITUTION: This patch antenna consists of a square external patch P1 formed on a dielectric substrate D, an internal patch P2 of an outer slit loading type which is formed on the substrate D in the patch P1, and the feeding parts F1 and F2 of both patches P1 and P2 respectively. The patch P2 includes 4 slits which are symmetrical to an in parallel to each other in the vertical direction with spaces secured among them. Then the same resonance frequency is set between both patches P1 and P2 so that the radiation power density of radio waves is improved in comparison with the ordinary patch antennas which have the same external shape size. Furthermore the wide band characteristic is secured when a slight difference of resonance frequency is set between both patches P1 and P2.