A model calculation of the cosmic ray Forbush decrease is performed on the basis of the diffusion-convection model. The main purpose is to evaluate the magnitude of the precursory increase expected from a model where the cosmic ray particles are swept away by traveling disturbances in which the solar wind speed is raised and/or the diffusion coefficient is reduced. It is found that the ratio of the precursory increase to the main decrease is less than ~10% if the field-aligned diffusion coefficient is larger than ~3¿1021 cm2/s at 1 AU and the speed of the solar wind disturbance is less than ~1000 km/s. Existence of a small precursory increase of this magnitude does not seem to contradict the observations of the Forbush decrease. The precursory increase can be >20% of the main decrease when the speed of the disturbance is >2000 km/s, however, and August 4, 1972, event seems to represent such a case.