A ceramic green sheet is irradiated with a laser beam of a high-peak short-pulse type to form a hole in the sheet. With the laser beam, a minimum oscillation output is more than 60% of a maximum oscillation output. This prevents material of the ceramic green sheet from melting partially and remaining around the hole. Therefore, a multi-layer assembly of ceramic green sheets built by joining the sheets in layers and baking these joined sheets hardly has a structural defect around a via-electrode provided in the hole.