The dynamics of cellulose diacetate (CDA) in semidilute solution (1.4c* < c < 5c*) was investigated in a good and high-polarity solvent, dimethylacetamide (DMAc), by dynamic light scattering in the quiescent state at 30 degrees C. Two fast diffusion motions and a very slow diffusion motion were detected, all the behavior of which is not expected from the framework of dynamics for usual flexible polymers in good solvents. The fast (fast mode) and medium-fast (medium mode) motions gave the diffusion coefficients D-F and D-M as D-F proportional to c(0.40) and D-M proportional to c(0.55). Both were the cooperative diffusion motions, or the relaxations of concentration fluctuations in two types of coarse-dense structures, which were realized as a nest of structures in a dynamic sense in semidilute solution. However, the relaxation rates were suppressed by the long-ranged hydrogen-bond interaction between hydroxyl (OH) groups on CDA molecules, and the interactions brought the systems into the medium, not good, solvent state. The slow diffusion (slow mode), which showed D-s proportional to c(-2.44), could be described as a self-diffusion, where the semiflexible CDA chain performs the reptation in a virtual tube that was constructed by other surrounding CDA chains. The reptation motion would be retarded, much more than neutral flexible chains, by the chain stiffness and by the attractive long-ranged hydrogen-bond interaction operating OH groups between the reptating CDA chain and the tube wall.