An image data generator for generating data on a size reduced image of an original image. In one embodiment, the image data generator includes: an inverse wavelet transform unit to perform on inverse wavelet transform on wavelet coefficients; and a coefficient selecting unit to select, of wavelet coefficients in a sub-band, wavelet coefficients to be subjected to the inverse wavelet transform. In one embodiment, the inverse wavelet transform unit performs the inverse wavelet transform only on the wavelet coefficients selected in the coefficient selecting unit with respect to the wavelet coefficients in the sub-band.