1. A method for blindly embedding a watermark into digital contents by using a human visual system (HVS) model and wavelet transform, the method comprising the steps of: (a) decomposing an original image by a wavelet and selecting a middle frequency band as a watermark embedment area; (b) obtaining an HVS model having a human visual recognition information at each embedment location; (c) adaptively determining the importance of coefficient value on each embedment location, combining the importance and the HVS model, and adaptively determining a quantization step for each embedding location; (d) quantizing each coefficients pair of embedment area by the quantization step and variably embedding a watermark sequence into a middle frequency band according to a value of the watermark; and (e) performing inverse wavelet transform on an overall area into which the watermark sequence is embedded, to generate a watermarked image.