Many research projects on software estimation use software size as a major explanatory variable. However, practitioners sometimes use the ratio of effort for early phase activities such as planning and requirement analysis, to the effort for the whole development phase of the software in order to estimate effort. In this paper, we focus on effort estimation based on the effort for early phase activities. The goal of the research is to examine the relationship of early phase effort and software size with software development effort. To achieve the goal, we built effort estimation models using early phase effort as an explanatory variable, and compared the estimation accuracies of these models to the effort estimation models based on software size. In addition, we built estimation models using both early phase effort and software size. In our experiment, we used ISBSG dataset, which was collected from software development companies, and regarded planning phase effort and requirement analysis effort as early phase effort. The result of the experiment showed that when both software size and sum of planning and requirement analysis phase effort were used as explanatory variables, the estimation accuracy was most improved (Average Balanced Relative Error was improved to 75.4% from 148.4%). Based on the result, we recommend that both early phase effort and software size be used as explanatory variables, because that combination showed the high accuracy, and did not have multicollinearity issues.