1. A scalable video encoding apparatus comprising: a first encoding unit to generate a base layer bitstream, and to generate a reconstructed image having a second resolution, by encoding an original image having a first resolution into an encoded image having the second resolution; and a second encoding unit to generate an enhancement layer bitstream by reconstructing the reconstructed image having the second resolution, which is received from the first encoding unit, to be a reconstructed image having the first resolution in response to an interpolation mode signal, and then to encode the original image having the first resolution by using the reconstructed image having the first resolution, where the enhancement layer bitstream contains additional information for generating a reconstructed image having the first resolution or a third resolution.