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A simple model is constructed to evaluate the change of star formation rateof a disk galaxy due to environmental effects in clusters of galaxies. Threeeffects, (1) tidal force from the potential well of the cluster, (2) increaseof external pressure when the galaxy plows into the intracluster medium, (3)high-speed encounters between galaxies, are investigated. General analysisindicates that the star formation rate increases significantly when thepressure of molecular clouds rises above $\sim 3\times 10^5 cm^{-3} K$ in $\sim10^8$ yr. The tidal force from the potential well of the cluster increasespressures of molecular clouds in a disk galaxy infalling towards the clustercenter. Before the galaxy reaches the cluster center, the star formation ratereaches a maximum. The peak is three to four times larger than the initialvalue. If this is the main mechanism of the Butcher-Oemler effect, bluegalaxies are expected to be located within $\sim 300$ kpc from the center ofthe cluster. However this prediction is inconsistent with the recentobservations. The increase of external pressure when the galaxy plows into theintracluster medium does not change star formation rate of a disk galaxysignificantly. The velocity perturbation induced by a single high-speedencounter between galaxies is too small to affect star formation rate of a diskgalaxy, while successive high-speed encounters (galaxy harassment) trigger starformation activity because of the accumulation of gas in the galaxy center.Therefore, the galaxy harassment remains as the candidate for a mechanism ofthe Butcher-Oemler effect.Comment: 12 pages, 13 figures. To be published in Ap

Quantitative Estimates of Environmental Effects on the Star Formation Rate of Disk Galaxies in Clusters of Galaxies