Abstract: A method of producing a gypsum plaster board. This method includes a step of preparing a starch paste which is effective to bond the gypsum core member and both paper boards. In this step, first, about 10-30 wt % of starch such as corn starch based on water is added to water and mixed to obtain milk of starch. Next, about 0.1-1.5 wt % of amylolytic enzyme such as .alpha.-amylase based on the starch is added and then heated to the decomposition point of the starch with stirring. After the heating is stopped at the range of DH 0.1-10, about 0.8-1.0 wt % of an enzyme deactivating agent based on the starch is added and mixed with water to prepare a starch paste. After the starch paste being prepared by the aforementioned process, a raw material of the gypsum core member, that is, calcined gypsum, reinforcing agent such as glass fiber, weight reducing agent such as surface active agent and the starch paste are mixed with water to obtain gypsum slurry.