Direct coupling among antenna elements is one of the critical problems of array antennas in near-field imaging systems. To mitigate the coupling, we propose taper-walled linearly tapered slot antenna (taper-walled LTSA) to be used in our adaptive ground penetrating radar (GPR) system for visualization of plastic landmines. It is based on the conventional walled LTSA, but the sidewall edges are curved at the aperture and connected to side slits. We analyze the characteristics of the proposed antenna in simulation, and demonstrate in experiment the reduction of the direct coupling in a wideband from 7.2 to 12 GHz which covers the observation frequency of our stepped-frequency adaptive landmine visualization system. The idea is directly usable also in other frequency bands.