Association of the active region NOAA 10720 magnetic flux and flare appearance is investigated. Distributions of the normal magnetic component in the active region are calculated in the potential approximation using SOHO MDI line-of-sight magnetic measurements. During traveling along the solar disk the magnetic flux of the active region increases up to 4×10 22 Mx. The X class solar flares appear, when the magnetic flux reaches ∼3×1022Mx. It is shown that the magnetic flux and the magnetic field distribution in the active region in the long duration X3.8 flare January 17, 2005 do not reveal considerable change. During the flare X3.8 magnetic energy flow from the photosphere is not observed. The obtained results show that flare energy released in the corona has been accumulated during the preflare time. This conclusion supports the flare model based on the conception of flare energy accumulation in the magnetic field of a coronal current sheet.