The present invention includes (a) dividing the reservoir in a coalbed methane (CBM) development area through computer modeling and thereby forming a three-dimensional grids having a plurality of cell, (b) actually forming drilling holes in the development area and thereby obtaining core samples for each depth, (c) modeling the amounts of ash, water, and gas in each of the cells, (d) modeling the pressure change over time in each of the cells under a premise in which a production well is formed in the grids, and (e) performing a Langmuir experiment for the core samples, determining a Langmuir function for a maximum amount of gas adsorption with pressure and amounts of ash and water as variables, and applying the Langmuir function to each of the cells to calculate an amount of gas detected due to pressure change over time.