A numerical study demonstrates that an ultrahigh extinction ratio of ≥3×10^9 is possible in a metasurface polarizer. The metasurface has stacked complementary structures with an air-hole array. From Babinet’s principle, this kind of metasurface is known to have a high extinction ratio exceeding 10^4. In this paper, numerical calculations reveal that Babinet’s principle incorporating destructive interference between localized and propagating surface plasmon polariton resonances is key in the enormous enhancements in the extinction ratio. Moreover, this study shows that an asymmetry in the complementary structures plays a crucial role in the fine adjustments of the interference. As a result, an extremely low transmittance is realized, leading to an ultrahigh extinction ratio of ≥3×10^9 in the O-band. An analytical study confirms that the low transmittance associated with ultrahigh extinction ratios originates from the destructive interference. Such ultrahigh performances are wavelength-tunable by a simple scaling of the metasurface and a fine adjustment of the interference. In the C-band, the ultrahigh extinction ratio exceeds 200×10^9. This study reveals the numerical and theoretical design criteria for ultrahigh extinction ratios in metasurface polarizers.