In this paper, we investigate the link level performance considering the influence of the error vector magnitude (EVM) of a transmit signal in a downlink non-orthogonal multiple access (NOMA) with two users. The motivation of this work is that to clarify the impact of the distortion of the transmit signal, which is expressed as the EVM, on the reception performance of each user equipment (UE). Results show that the performance of the near-base-station UE, which implements a successive interference cancellation (SIC), is deteriorated as the EVM increases, and that the amount of the deterioration rapidly increases as the modulation level is higher. In contrast, the performance of the far-base-station UE, which does not implement the SIC, is less affected even with the large EVM. Furthermore, we also show that the downlink NOMA requires lower EVM values than the downlink orthogonal multiple access (OMA), when comparing them at the same modulation level. Accordingly, a high quality radio frequency (RF) transmitter will be required for the introduction of NOMA to the next generation mobile communication systems.