An enzymatic reaction model, in which the enzymes are fixed at respective spatial positions and the moving reactants drived by free diffusion react with the fixed enzymes, was built to simulate a metabolism in cellular conditions. Further, it was assumed in the model that the final product inhibits the first enzyme by the negative feedback. The dynamical behavior of the model was surveyed by computer simulation to evaluate its capability of the constant-value control or homeostatic control. It was found as a result that the model system exhibited a weak controllability on the homeostasis of the metabolites against the external perturbation added to the system. Some discussions were also made on the mechanism of homeostatic control of metabolites under cellular conditions.