An inventive method includes the steps of: loading a fiber product in an appropriate form into a treatment vessel; wetting the fiber product; forcibly circulating an ozone-containing liquid in contact with the wetted fiber product to thereby bleach the fiber product; and forcibly circulating an ozone decomposing chemical agent liquid in contact with the ozone-treated fiber product to decompose ozone. This method ensures efficient bleaching of the fiber product with the ozone. Further, the method is advantageous in that the resulting bleached fiber product is less liable to be yellowed over time.