As continuous wave (CW) terahertz (THz) sources, the differential-frequency-mixing (DFM) has an advantage for the frequency tunability by changing the energy separation of the two lasers. In particular, considering the inhomogeneous width in the quantum conﬁnement systems, use of the exciton lines enables wide frequency tuning. The THz sources with the narrow bandwidth and wide frequency tunability will be applied to the high resolution THz spectroscopy. Recently, we realized the CW-THz wave generation by DFM under the exciton excitation conditions in a GaAs/AlAs multiple quantum well (MQW), which shows the wide frequency tuning range over 18 THz. Therefore, in this work, we report the polarization characteristics of a continuous THz electromagnetic wave generated by DFM due to excitation of two exciton states in the GaAs/AlAs multiple quantum well. As a sample, we used an undoped GaAs/AlAs MQW embedded in a p-i-n structure on a (001) n+-GaAs substrate. The thickness of GaAs and AlAs layer is 7.5 nm. The measurements of the THz wave were carried out at 296 K. As the laser sources, a semiconductor laser and a CW-mode Ti:sapphire laser to change the excitation energy were used. The two beams were focused on the sample surface. Comparing the polarization of the laser beams with that of the THz wave, the conversion process from the laser lights to the THz wave via the exciton states, such as the heavy hole and light hole excitons split by quantum confined effects, will be demonstrated.