A method of controlling washing operation of a washing machine having a tub containing water, a drum rotatably installed in the tub, a heater installed in the tub to heat water supplied to the tub, and a water supply pipe to guide water supplied from a water pipe through a water supply valve into the tub, comprising the steps of: washing a laundry, the washing laundry including: supplying water into the tub, the water being supplied into the tub such that a bottom of the drum is immersed in the water (S110); soaking the laundry with the water by rotating the drum (S120); after completing soaking the laundry, generating steam by heating water in the tub using the heater , and supplying the steam into the drum through apertures of the drum (S170); rinsing the laundry after the washing is finished; and spin-drying the laundry after the rinsing is finished, characterized in that after completing soaking the laundry and before starting generating steam, discharging water in the tub through a water drain pipe in case a water level in the tub after the soaking the laundry exceeds a preset high water level (L1) sensed via a water level sensor (181-183) for controlling the water and required for a generation of steam (S150) and re-supplying water in the tub through the water supply pipe in case the water level in the tub after the soaking the laundry is below the preset high water level (L1) for controlling the water level in the tub (S140), wherein, at the re-supplying the water, the water is supplied in the vicinity of a storage part provided at a bottom of the tub to allow the water to be supplied to the storage part without passing the laundry, and wherein the preset high water level (L1) is a level that allows the heater to be submerged in the water, while not allowing a bottom of the drum to be submerged in the water. washing a laundry, the washing laundry including: supplying water into the tub, the water being supplied into the tub such that a bottom of the drum is immersed in the water (S110); soaking the laundry with the water by rotating the drum (S120); after completing soaking the laundry, generating steam by heating water in the tub using the heater , and supplying the steam into the drum through apertures of the drum (S170); rinsing the laundry after the washing is finished; and supplying water into the tub, the water being supplied into the tub such that a bottom of the drum is immersed in the water (S110); soaking the laundry with the water by rotating the drum (S120); after completing soaking the laundry, generating steam by heating water in the tub using the heater , and supplying the steam into the drum through apertures of the drum (S170); rinsing the laundry after the washing is finished; and spin-drying the laundry after the rinsing is finished, characterized in that after completing soaking the laundry and before starting generating steam, discharging water in the tub through a water drain pipe in case a water level in the tub after the soaking the laundry exceeds a preset high water level (L1) sensed via a water level sensor (181-183) for controlling the water and required for a generation of steam (S150) and re-supplying water in the tub through the water supply pipe in case the water level in the tub after the soaking the laundry is below the preset high water level (L1) for controlling the water level in the tub (S140), wherein, at the re-supplying the water, the water is supplied in the vicinity of a storage part provided at a bottom of the tub to allow the water to be supplied to the storage part without passing the laundry, and wherein the preset high water level (L1) is a level that allows the heater to be submerged in the water, while not allowing a bottom of the drum to be submerged in the water. <EOS>