PROBLEM TO BE SOLVED: To provide a production method of an inorganic fertilizer capable of mineralizing an organic fertilizer in a short period and efficiently recovering nitrate nitrogen without adding microorganisms.SOLUTION: A production method of a microorganism carrier decomposing organic material to nitrate nitrogen comprises the steps of: (a) adding organic material to an unfired shell and standing them for 8 hours or more, thereby activating microorganisms attached to the shell； (b) adding water to the shell and making water flow out of the shell to wash the shell； (c) and measuring an amount of nitrate nitrogen in the outflow liquid flown out of the shell in the step (b), repeating the steps (a) and (b) if nitrate nitrogen is not generated in the outflow liquid, and obtaining the shell as a microorganism carrier if nitrate nitrogen is generated in the outflow liquid. In a production method of an inorganic fertilizer comprising nitrate nitrogen, organic material is decomposed using a catalyst column which uses the microorganism carrier as a column carrier and decomposes organic material to nitrate nitrogen.
