Provided are a method and apparatus for enhancing coding efficiency by performing encoding and decoding by optimally calculating quantization intervals and reconstruction values through a distribution of DCT coefficients of each frame, when DCT coefficients of each SNR enhancement layer are quantized in scalable video coding. The encoding apparatus includes: an R-D optimization unit performing Rate-Distortion (R-D) optimization based on a distribution of Discrete Cosine Transform (DCT) coefficients each slice and calculating a first reference value and a second reference value respectively indicating a start point and an end point of DCT coefficients quantized to “0”; a quantization interval setting unit setting adaptive quantization intervals on the basis of a minimum value and a maximum value of the DCT coefficients, the first reference value, and the second reference value; and a mapping unit mapping the DCT coefficients to the adaptive quantization intervals.