NOVELTY - Ammonia concentration decreasing apparatus which reduces ammonia concentration in fermented liquor in fermenter (1), and has ammonia removal apparatus (5) having diaphragm (23), fermented liquor circulation unit (30), and acidic solution supply unit (8). Unit (30) connects chamber (25) of diaphragm and fermenter. Unit (8) supplies acidic solution to chamber (27) of diaphragm. Liquor is contacted with acidic solution through diaphragm, and ammonium ion is passed and transferred to acidic solution without permeating suspension component in liquor through diaphragm.    USE - The apparatus is used for reducing ammonia concentration in fermented liquor (claimed) produced in biogas production system used for fermenting organic waste and producing biogas. Organic waste includes organic refuse, wastewater treatment sludge, livestock farming waste material of pig, cow, sheep, goat and hen, and green farming waste material.    ADVANTAGE - The apparatus stably enables reduction of ammonia concentration in fermented liquor at high temperature.    DETAILED DESCRIPTION - The ammonia concentration decreasing apparatus of fermented liquor reduces ammonia concentration in fermented liquor contained in fermenter (1). The apparatus has an ammonia removal apparatus (5) having diaphragm (23), a fermented liquor circulation unit (30), and acidic solution supply unit (8). The circulation unit connects chamber (25) of one side of diaphragm, and fermenter. The unit (8) supplies acidic solution to chamber (27) of other side of diaphragm. The fermented liquor is contacted with acidic solution through diaphragm, and ammonium ion is passed through diaphragm and transferred to acidic solution without permeating suspension component contained in fermented liquor through diaphragm.    An INDEPENDENT CLAIM is included for ammonia concentration reduction method of fermented liquor.    DESCRIPTION OF DRAWING(S) - The figure shows the outline block diagram of biogas production system.    Fermenter (1)    Ammonia removal apparatus (5)    Acidic solution supply unit (8)    Diaphragm (23)    Chambers (25,27)    Fermented liquor circulation unit (30)