1. A sequential pattern extracting apparatus which extracts a sequential pattern frequently appearing in a plurality of sequential data each including items, comprising: a first storage device to store constraining sequential data expressing sequential data which a sequential pattern to be extracted must include; a generating device configured to generate a sequential pattern candidate set from the plurality of sequential data, the sequential pattern candidate set including a plurality of sequential pattern candidates each including sequential data; a second storage device to store the sequential pattern candidate set; a first elimination device configured to eliminate sequential pattern candidates which do not satisfy sequential pattern candidate conditions from the sequential pattern candidate set stored in the second storage device, the sequential pattern candidate conditions specifying inclusion relations of the items between the sequential data included in the sequential pattern candidates and the constraining sequential data; and an extracting device configured to extract the sequential pattern including the constraining sequential data from the sequential pattern candidate set remaining in the second storage device.