An efficient design method for plasmonic devices composed of a periodic slit array based on extraordinary optical transmission is proposed using Q factor analysis. The proposed method significantly reduces the required computation time for cavity resonance in slits by calculating the Q factor at a single frequency that is an eigenfrequency of the wave equation instead of computing the entire transmissions. The computation time is reduced by 20.60 times, and the device performance is increased by 2.43 times through a design based on Q factor analysis. The figure of merit for the overall performance is defined as a product of the transmission peak value and the Q factor. The design variables are the slit width, slit height, and period of slit array. The optical field distributions are calculated numerically using the finite element method, and the Q factor is obtained by solving an eigenfrequency for the wave equation for dispersive media.