The imaginary part of optical potentials for fixed exciton number to be used for preequilibrium reactions is microscopically calculated. The semiclassical approximation and a simple delta function type interaction are adopted. Only the lowest order processes are taken into account, and the incident particle is restricted to nucleons. Five types of processes contribute to the imaginary potential; the first is the creation of a particle-hole pair by the incident nucleon. This process is considered in usual optical potentials, but in this work the intermediate states are restricted to Q space. The imaginary parts corresponding to the other four processes are obtained in analytic forms, to which the effects of particle-hole states in the target nucleus are taken into account. The resultant imaginary parts are, with good accuracy, independent of the excitation energy and related to the exciton number in a very simple way. The transmission coefficients are calculated using obtained imaginary parts and compared with the ones based on conventional imaginary potentials. The effect of inelastic scattering before absorption is studied in a perturbation expansion and the second-order terms are found to be comparable with the zeroth-order terms.