It has long been known that some spiral galaxies have a large-scale lopsided structure (e.g., Baldwin, Lynden-Bell, & Sancisi 1980). The frequency of the lopsidedness in disk galaxies reaches to half of H i disks (e.g., Richter & Sancisi 1994) and one third of stellar disks (e.g., Zaritsky & Rix 1997). The fraction of lopsided disks is large, nevertheless their origin is not understood well. Although some theoretical models are proposed to explain the lopsidedness, dynamical friction between lopsided disks and spherical halos has not been taken into account. Then, in this contribution, we have examined the effect of dynamical friction on lopsided disks. To avoid discreteness noise due to small number of particles on the estimate of dynamical friction, we have used the matrix method.