To provide a production method of an inorganic plate capable of surely preventing a generation of efflorescence.

A hydraulic material consisting essentially of cement is formed to prepare a green sheet, and the green sheet is subjected to autoclave aging to produce the inorganic plate having 0.95-1.3 specific gravity. In the production of the inorganic plate like this, after applying a sealer in which a resinous water-based dispersion (a) having 0.06-0.09μm average grain size of a dispersoid and the resinous water based dispersion (b) having 0.10-0.20μm average grain size of the dispersoid are blended so that (a solid content of the resinous water- based dispersion (a)/the solid content of the resinous water-based dispersion (a) + the solid content of the resinous water based dispersion (b)) may be a wt. ratio of 0.6-0.8 on the surface of the green sheet, the sheet is subjected to the autoclave aging.