1. A moving object contour extraction apparatus for extracting from a three-dimensional image time series contours of a moving object which performs a deforming motion, the three-dimensional image time series comprising a plurality of three-dimensional images acquired at a plurality of time points, each of the three-dimensional images consisting of a plurality of parallel two-dimensional image slices, and the two-dimensional image slices located at the same location in the plurality of three-dimensional images forming an image slice time series, the apparatus comprising: a contour acquisition unit configured to acquire a contour of the moving object in each image slice; and a contour correction unit configured to correct the contours of the moving object in the image slices of at least one image slice time series based on motion trend information of the moving object in each of a plurality of the image slice time series.