Magnetizing process of a sediment is formulated using a linear system theory. Remanent magnetization is represented as a convolution integral of geomagnetic field variation and a moment fixing function. The formulated model predicts that a field variation is deformed by filtering to be recorded in magnetization of sediments, and that the original field is deduced from the record of magnetization by a deconvolution method. Application of the model to the magnetization data of a core of shallow water sediment reveals that the remanent intensity variation may reflect the intensity reduction due to the cancellation of magnetic moments aligned in various directions rather than decrease in a field intensity. Width of magnetizing zone is inferred to be unexpectedly small from results of measurement of magnetization and laboratory experiments. The half fixing depths, at which a half of magnetic moments in a horizon is locked in, seem to cluster around 10cm or less.