A hydrogen detector for gas and fluid media which comprises a housing comprising: a ceramic sensing element made of solid electrolyte which is designed as a cylinder interlinked with the bottom located in the lower part of the detector and comprises an inner cavity, wherein an external cylindrical surface of the ceramic sensing element is tightly connected to an inner side surface of the housing and wherein a layer of porous platinum electrode is applied to the external part of the ceramic sensing element bottom, a potential measuring unit comprising a central core having an end extending inside the ceramic sensing element, a reference electrode located in the inner cavity of the ceramic sensing element, a sealed lead-in tightly fixed inside the housing above the ceramic sensing element, a lower bushing shaped as a tube, connected to and extending below a lower part of the housing on the side of the ceramic sensing element, wherein the lower end of the lower bushing has a bottom with a center hole, and a selective membrane made of at least one tube with a lower free end tightly closed with a plug, the membrane attached by an upper free end to the center hole of the lower bushing; wherein the potential measuring unit that passes through a central core of the sealed lead-in and the lower bushing, so that the central core of the potential measuring unit is in electrical contact with the reference electrode; wherein a leak-proof cavity is limited by an inner surface of the lower bushing, by an external part of the bottom end of the ceramic sensing element, the inner surfaces of the selective membrane and the plug; wherein the hydrogen detector has an upper bushing installed in the upper part of the potential measuring unit; wherein a ring-shaped cavity formed between the inner surface of the upper bushing wall and the external surface of the central core of the potential measuring unit is filled with glass-ceramic sealant comprising: silicon oxide (SiO 2) 45-55 weight %, aluminum oxide (Al 2 O 3) 4-6 weight %, boric oxide (B 2 O 3) 18-22 weight %, titanium oxide (TiO 2) 9-12 weight %, sodium oxide (Na 2 O) 12-15 weight %, potassium oxide (K 2 O) 1-2 weight %, and magnesium oxide (MgO) 2-3 weight %; whereby the sealant fills the ring-shaped cavity and the potential measuring unit to form a leak-tight seal; and wherein, when placing the hydrogen detector in a medium, hydrogen in the medium reversibly diffuses through the selective membrane into the leak-proof cavity bounded by the inner surface of the lower bushing, the external surface of the ceramic sensing element and an inner surface of the selective membrane. a ceramic sensing element made of solid electrolyte which is designed as a cylinder interlinked with the bottom located in the lower part of the detector and comprises an inner cavity, wherein an external cylindrical surface of the ceramic sensing element is tightly connected to an inner side surface of the housing and wherein a layer of porous platinum electrode is applied to the external part of the ceramic sensing element bottom, a potential measuring unit comprising a central core having an end extending inside the ceramic sensing element, a reference electrode located in the inner cavity of the ceramic sensing element, a sealed lead-in tightly fixed inside the housing above the ceramic sensing element, a lower bushing shaped as a tube, connected to and extending below a lower part of the housing on the side of the ceramic sensing element, wherein the lower end of the lower bushing has a bottom with a center hole, and a selective membrane made of at least one tube with a lower free end tightly closed with a plug, the membrane attached by an upper free end to the center hole of the lower bushing; wherein the potential measuring unit that passes through a central core of the sealed lead-in and the lower bushing, so that the central core of the potential measuring unit is in electrical contact with the reference electrode; wherein a leak-proof cavity is limited by an inner surface of the lower bushing, by an external part of the bottom end of the ceramic sensing element, the inner surfaces of the selective membrane and the plug; wherein the hydrogen detector has an upper bushing installed in the upper part of the potential measuring unit; wherein a ring-shaped cavity formed between the inner surface of the upper bushing wall and the external surface of the central core of the potential measuring unit is filled with glass-ceramic sealant comprising: silicon oxide (SiO 2) 45-55 weight %, aluminum oxide (Al 2 O 3) 4-6 weight %, boric oxide (B 2 O 3) 18-22 weight %, titanium oxide (TiO 2) 9-12 weight %, sodium oxide (Na 2 O) 12-15 weight %, potassium oxide (K 2 O) 1-2 weight %, and magnesium oxide (MgO) 2-3 weight %; silicon oxide (SiO 2) 45-55 weight %, aluminum oxide (Al 2 O 3) 4-6 weight %, boric oxide (B 2 O 3) 18-22 weight %, titanium oxide (TiO 2) 9-12 weight %, sodium oxide (Na 2 O) 12-15 weight %, potassium oxide (K 2 O) 1-2 weight %, and magnesium oxide (MgO) 2-3 weight %; whereby the sealant fills the ring-shaped cavity and the potential measuring unit to form a leak-tight seal; and wherein, when placing the hydrogen detector in a medium, hydrogen in the medium reversibly diffuses through the selective membrane into the leak-proof cavity bounded by the inner surface of the lower bushing, the external surface of the ceramic sensing element and an inner surface of the selective membrane.