The mask fit tester is a device that measures the fitness of a mask to be used to the face. A Condensation particle counter has generally used to measure the number concentrations, and a method of measuring the fit factor is performed by dividing the number of particles outside the mask by the number of particles inside. Although NIOSH’s mask-fit tester for the N95 standard uses DMA to measure only particles smaller than 100 nm to avoid the effects of passing particles on the face piece itself. Among the mask testers for the N95 standard, the AccuFIT 9000 PRO (Kanomax Japan) uses a small and simple parallel plate DMA. In this study, the performance of the transfer function of this parallel plate DMA was evaluated by the tandem DMA method. The transfer function of the reference DMA used for this evaluation was obtained by the method of simultaneously evaluating the transfer functions of three DMAs. As a result, it was found that the transfer function of the parallel plate DMA had a wider transfer function. However, it was clarified that the parallel plate DMA has sufficient classification performance for the application of mask fit testing to the N95 standard.