A device for coding picture data by compression, and a method thereof are provided. By preprocessing, the device calculates a picture activity of one frame of picture data. A quantizing table circuit selects a particular quantizing step matching the picture activity and sets the picture activity in a quantizing circuit. On a first path, one frame of picture data is fed to a discrete cosine transform (DCT) circuit to be thereby subjected to orthogonal transform. The quantizing circuit sequentially quantizes DC components, low frequency components and high frequency components of the resulting output of the DCT circuit in ascending order of frequency block by block according to a quantizing step set therein. The picture data from the quantizing circuit are coded by Huffman coding while a code counter counts the coded data. The quantizing step set in the quantizing circuit is updated on the basis of the amount of coded data and a target amount of codes. On a second path, the same one frame of picture data is again fed to the DCT circuit to undergo orthogonal transform and compression, as on the first path. The coded picture data are sent to a memory card or similar picture data storing medium. A bit distributing circuit produces a bit distribution value block by block. When the output count of the code counter associated with one block reaches the bit distribution value, the DCT circuit, quantizing circuit and Huffman coding circuit are disenabled while an EOB (End of Block) code is added to the end of the block of coded picture data.