To obtain better statistics on the occurrence of magnetism among white dwarfs, we searched the spectra of the hydrogen atmosphere white dwarf stars (DAs) in the Data Release 7 of the Sloan Digital Sky Survey (SDSS) for Zeeman splittings and estimated the magnetic fields. We found 521 DAs with detectable Zeeman splittings, with fields in the range from around 1 to 733MG, which amounts to 4% of all DAs observed. As the SDSS spectra have low signal-to-noise ratios, we carefully investigated by simulations with theoretical spectra how reliable our detection of magnetic field was.