[en] The dynamical interaction between the intergalactic gas and the interstellar gas of a disk galaxy is calculated by a time-dependent, two-dimensional hydrodynamic code. The case where the velocity vector of intergalactic gas is parallel to the rotational axis of the galaxy is examined. The gas ablation from the disk is completed within 10⁸ years, and the small gas disk remains at the bulge of a disk galaxy. A simple analytic expression for the radius of this remnant gas disk is derived from the calculated results, and the gas ablation time is estimated. It is concluded that spiral galaxies are transformed to SO or anemic galaxies under the general conditions of intergalactic gas within a rich cluster of galaxies. (author)