THz imaging, which is a THz wave spectroscopic diagnosis, was developed to solve the weakness of existing imaging techniques. One of the THz imaging techniques is THz near-field imaging. However, this technique has an issue that is only able to capture breast tissue image with the thickness of 20 μm. Optimum use of a proper antenna is expected to solve this problem. In this paper, we propose a rectangular patch microstrip array antenna design modified with inset line and rectangular slot to image thicker breast tissue. The antenna has a center frequency, bandwidth, gain, horizontal beamwidth and vertical beamwidth of 0.312 THz, 22.68 GHz, 5.6 dB, 86.5 degrees and 47.1 degrees, respectively. This antenna is expected to be able to image the breast tissue with the thickness of 2 mm and the distance between the transmitter and the receiver antenna of 4.5 mm.