Two types of laboratory-scale experiments were conducted to investigate nitrate removal by sulfur oxidizing denitrifiers fed with thiosulfate as electron donor. One was a batch-type activity test of denitrification with glass vials using enrichment culture. The other was a downflow column experiment at 20°C and at a filtration velocity of 4m·day^-1, in which artificially contaminated water was supplied to packed-bed filled with granular anthracites. The stoichiometries of denitrification from nitrate and nitrite by oxidation of thiosulfate were discussed with removal and production data in several batch tests. The conversion rate from nitrate to nitrite was much higher than from nitrite to nitrogen gas in the batch test. The nitrogen concentration profile along the column showed that rapid removal of nitrate was observed at the upper part of the filter after development of biofilm in bed, although remarkable accumulation of nitrite took place there. The accumulation was reduced downward to some extent by distribution of attached biomass to the lower part. However, nitrite in the effluent was not removed completely after ammonium was added as an easily assimilable nitrogen source in order to promote the activity and growth of sulfur-denitrifiers. There was a possibility that their activity and growth was not controlled by nitrogen source for synthesis but suppressed by lower pH condition within the biofilm than bulk water in the column.