Motion detecting method and motion detector

A motion detecting method and a motion detector are provided. The motion detecting method includes the following steps. When the type of the current macro block (MB) is intra-type (I-type) or predictively-coded type (P-type), a first procedure or a second procedure is performed. The first procedure includes setting the active flag of the currentMB according to the type of the previous MB. The second procedure includes setting the active flag of the current MB according to the motion vectors of the previous MB and the current MB. The present invention is capable of reducing the probability of erroneous motion judgments.