A method for estimating a man-hours of an entire project having a series of tasks with a computer includes, inputting an estimated man-hours of the each task, acquiring model functions for extracting estimation errors included in the estimated man-hours of the each task based on an attribute of a worker who performs the each task, calculating a probability density distribution representing estimation errors depending on the attribute and a probability density distribution representing modeling errors depending on methods for estimating the man-hours for each task using the model functions, calculating man-hours of the entire project having a series of tasks for the each task using statistical methods to accumulate the probability density distribution representing estimation errors and the probability density distribution representing the modeling errors, and outputting calculating results of man-hours of the entire project to a output device.