The dynamics of polystyrene (PS)-block-poly(methyl methacrylate)(PMMA) in the semidilute solution region was studied by dynamic light scattering measurements made on the benzene solutions of polymer mass concentration c = 0.844c*-2.00c* at 30-degrees-C. The dynamic structure factor observed for the visible PS parts was expressed at all c and in 0.13 < qR(G) < 1.54 by the sum of two or three exponentials whose decay rates GAMMA(i) were separated distinctly from each other. For the slow mode of the smallest GAMMA values, GAMMA was proportional to q2 and the motion was assigned to the center-of-mass (or the long-time) diffusion motion of a single block chain in a pregel state, but the diffusion coefficient showed a change in its c dependence from c-1 to c-1-75 with increasing c. For the medium mode, GAMMA was also proportional to q2 and the diffusion coefficient value was about 4 times larger than that of the slow mode. This feature allowed us to assign this motion to be a cooperative diffusion of the PS concentration blob (the size xi(H)), but the c dependence of D(coop,PS) is-proportional-to c-1 at higher c was found to disagree with the scaling prediction in a good solvent, D(coop) is-proportional-to c0.75. The fast mode of the largest GAMMA values was independent of c but the GAMMA values showed a q2 dependence at (qR(G))2 < 1.2 and a q4-5 dependence at (qR(G))2 > 1.3. These behaviors indicated that the former represents the relative motion of PS parts with respect to PMMA parts and the latter the Rouse dynamics, and totally the interdiffusion motion. In addition, there were some indications of the coupling between the cooperative diffusion and interdiffusions. The first cumulant GAMMA(e), which shows the short-time behavior of the chain, was found to exhibit a clear transition from a rough q2 dependence to a q4 dependence at qxi(H) congruent-to 0.6. This means that even in a good solvent the internal motion of the diblock chain in the semidilute region can be represented by Rouse dynamics at qxi(H) greater-than-or-equal-to 0.6.