In this study, we reconsider the optimal nonlinear tax problem with the public good from the perspective of the commitment issue and examine how it affects the condition of the public good provision. We show that the Samuelson rule should be modified when the government cannot commit and the skill types of taxpayers are revealed in the first period. This is true even if the preference of the taxpayers is separable and additive with respect to consumption and leisure. Our analysis also shows how the lack of commitment affects the formula of the marginal cost of public funds and the level of public good provision. Our findings imply that the level of the public good may be excessive in comparison to the case where the government can commit to its tax policy.