The temporal derivative has been considered as a mathematical solution for the latency variation of the hemodynamic response function (HRF) in the general linear model (GLM) analysis of the task-based functional MRI (fMRI). A method of combining the primary and derivate estimates was developed by Calhoun and its implementation was introduced. However, serious defects were revealed in the existing methods from a GLM analysis of an event-related fMRI. Here, the method is revised to provide a correct combined estimate using a weighted square average method. The proposed method was confirmed with event-related fMRI studies at various phases of the double Gamma HRF.