To provide an electronic watermark method and system for which the detection reliability of embedded information does not depend on the strength of signals that are measured in frames. To achieve the above object, information is prepared for embedding as a bit stream, the sign of the bit stream being changed in accordance with a sign inversion cycle, and the bit stream being embedded in a frame. As for detection of information, values obtained through observation of frames are accumulated, and the accumulated values are compared with threshold values that are varied in accordance with the accumulated values obtained by observation. The comparison results are then employed to detect the embedded information.