An image signal compressing device having a spatial frequency data amount setting unit and a quantization table generation unit. A set data amount is set in the spatial frequency data amount setting unit for each of a plurality of spatial frequencies based on a preset total data amount and a discrete cosine transformation (DCT) data statistical amount corresponding to quantized DCT coefficients using a quantization table in which all quantization coefficients are "1". An amount of encoded data obtained using a predetermined quantization coefficient is estimated in the quantization table generation unit. The predetermined quantization coefficient is adopted as a resultant quantization coefficient of the quantization table when the estimated data is not greater than the set data amount set in the spatial frequency data amount setting unit.