We have made a quantitative calculation for the systematic evolution of average extinction by interstellar dust in host galaxies of high-redshift Type Ia supernovae, by using a realistic model of photometric and chemical evolution of galaxies and supernova rate histories in various galaxy types. We find that average B band extinction at z \sim 0.5 is typically 0.1-0.2 mag larger than present, under a natural assumption that dust optical depth is proportional to gas column density and gas

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... city. This systematic evolution causes average reddening with E(B-V) \sim 0.025-0.05 mag with the standard extinction curve, and this is comparable with the observational uncertainty of the reddening of high-redshift supernovae. Therefore, our result does not contradict the observations showing no significant reddening in high-z supernovae. However, the difference in apparent magnitude between an open universe and a \Lambda-dominated flat universe is only \sim 0.2 mag at z \sim 0.5, and hence this systematic evolution of extinction should be taken into account in a reliable measurement of cosmological parameters. Considering this uncertainty, we show that it is difficult to discriminate between an open and \Lambda-dominated flat cosmologies from the current data.