In order for Terahertz (THz) technology to enter the forefront of the industry, technology to generate THz wave with simple structure and low cost is required. This paper suggests compact and portable THz imaging system by the use of Single Mode Fabry-Perot Laser Diodes (SMFPLD). The existing THz system has a disadvantage that it is bulky, complicated, expensive, and difficult to use. To compensate for this, we use an inexpensive and simple FPLD combined with an external cavity to construct a THz imaging system. SMFPLD shows a steady longitudinal mode state through self-injection locking by means of a built-in aspherical lens. The position of the dominant mode changes as the current and temperature are controlled, and the dual mode is also generated under certain conditions. This characteristic is a great advantage in the photomixing method that requires two lasers. Because a laser alone can create a THz wave, and it creates two frequencies in a single family, it is not necessary to consider polarization or phase differences. Based on these advantages, we have constructed a laser module and combined with PCA to create a THz imaging system. A THz imaging systems demonstrated in this paper are superior to ordinary systems due to their inexpensive and compact structure.