The rod and cone cells have a common phototransduction mechanism. First, the visual pigments are stimulated by absorbing light and activate a G-protein called transducin. This transducin subsequently activates phosphodiesterase, which in turn decreases the cGMP concentration. Subsequently, the cGMP sensitive current reduces, leading to depolarization. In this process, the activated visual pigments are phosphorylated several times by rhodopsin kinases. Although the phototransduction mechanisms are common with rod and cone cells, the response speed and the light sensitivity are considerably different. The phosphorylation rate of the activated visual pigments by the rhodopsin kinases and the transducin activation rate by the stimulated visual pigments are reported in detail for both rod and cone cells by Tachibanaki, Kawamura et al. [1]. In their report, the relation between the phosphorylation rate per activated visual pigment and light intensity shows strong negative exponential characteristics. Also, the relation between the activated transducin per activated visual pigments and the light intensity are not monotonous. A strong light activates less transducin compared to a moderate light. For the quantitative understanding and analysis of the phototransduction mechanism, a detailed model is proposed by Hamer et al. [2]. This includes the transducin activation process by activated visual pigments and the phosphorylation process of the visual pigments by rhodopsin kinase. However, the transducin time course reported by Tachibanaki et al. could not be reproduced by the model. Thus, we propose a modified phototransduction model. In our model, we made two modifications to the Hamer model: First, the calculation of the free rhodopsin kinase was refined. Second, the transducin activation by the activated visual pigment was modified such that it is dependent on the ratio of the activated visual pigments to the total visual pigments. With these modifications, the transducin production and the phosphorylation of the visual pigments under various light intensities were successfully reproduced. The analysis of the simulation results showed that the amount of the free rhodopsin kinase decreases with the increase in light intensity since the complex of the visual pigment and the rhodopsin kinase increase. This mechanism leads to the nonlinear relation between the phosphorylation rate of the activated visual pigments and light intensity. However, this modification could not reproduce the non-monotonous relation between the transducin activation and light intensity. Although the mechanism is not clear, by introducing the the nonlinear relation between the transducin activation and the visual pigment activation ratio, we could successfully reproduce the non-monotonous relation of the transducin activation.