The purpose of a series of our studies is to discuss effective method of mathematical modelling by using computer. In this paper, activities for mathematical exploration by using computer are focused. For this purpose, we treated Laplace equation and studied some basic properties of harmonic functions, Poisson integrals, and the Dirichlet problem for the unit disk. A feature of the method is to provide situations in which students make mathematical models. And another feature is to give students enough time to get the numerical calculation and make conjectures on some results by using computer. The practice shows that some students had difficulty in making and solving mathematical models by using computer, but gradually felt interested in considering about some phenomenon.