Abstract: An information source encoding system in which overload noise and block noise are less recognized and quantizing overload and large quantizing distortion can reduced within an intra-frame mode encoding region even when intra-frame and inter-frame mode encoding regions coexist in an image screen. An orthogonal converter outputs conversion coefficients in a predetermined spatial frequency region. A coefficient separating unit selectively separates the conversion coefficients. With the separated coefficient from the coefficient separating unit, a power is calculated. The calculated power is compared with a predetermined threshold value by a coefficient power determining unit.