Dynamics of cellulose diacetate (a typical polysaccharide) in N,N-dimethylacetamide (a hydrogen-bond enhancing solvent) at 27 degrees C was investigated in a shear field of velocity gradient gamma=0-5.3 s(-1) through dynamic light scattering under circular Couette flow. The polymer mass concentration was set around the crossover of dilute and semidilute regimes. Growth of extra-large length-scale fluctuations and their transitions with shear were detected, which were originated by the long-ranged interaction, or the hydrogen bond, operating on the OH and CO groups in the glucose residues of cellulose acetates via high electronegative solvent. Two modes of fluctuations with fast and slow decay rates Gamma(f) and Gamma(s) were induced and each Gamma value changed stepwise and linearly with gamma, respectively. The decay rate distribution, which was estimated from the electric-field time correlation function expressed in the KWW-type of stretched exponential decay form g((1))(t)=A exp[-(t/tau)(beta)] with beta the index of decay rate distributions, was changed from 1/2 (one-sided stable distribution) to 3/2 (Holtzmark distribution) around gamma=0.8-1 s(-1) for the fast mode, while beta=2 (Gaussian, or the sharpest distribution) over the full range of 0.40 0.4 s(-1). In addition, oscillatory feature was conspicuous in g((1))(t) at every gamma of 0.40 3.7 s(-1), which may come from the competition between shear flow distortion and the Brownian diffusion of polymer chain in solution and can be grasped qualitatively by the initial stage of spinodal decomposition under the control of long-ranged attractive interactions.