A control method of a washing machine comprising a tub for containing wash water, a drum rotatably mounted in the tub for containing laundry, and a motor for rotating the drum, comprising: a primary laundry amount determination step (S10, S10', S100) of measuring the amount of the laundry placed in the drum before supplying wash water to the tub; a data setting step (S20, S20', S200) of setting control data to control the washing machine based on the amount of the laundry measured at the primary laundry amount determination step; a secondary laundry amount determination step (S50, S50', S500) of measuring the amount of the laundry placed in the drum after supplying wash water to the tub; a data resetting step (S65, S631', S631) of resetting the control data based on the amount of the laundry determined at the secondary laundry amount determination step when the amount of the laundry measured at the primary laundry amount determination step is different from the amount of the laundry measured at the secondary laundry amount determination step; a washing step (S70, S70', S700) of rotating the drum in a predetermined rotation direction, the washing step being performed after the data resetting step; characterized in that, the secondary laundry amount determining step includes an observation motion step comprising rotating the drum to a reference angle in a direction opposite to the rotation direction set at the washing step to measure electric current supplied to the motor, the observation motion step being performed before the washing step, wherein the primary laundry amount determination step comprises rotating the drum to a predetermined reference angle to measure before-water-supply motor load, the secondary laundry amount determination step further comprises a before-water-supply motor load estimation step comprising estimating before-water-supply motor load from the measured after-water-supply motor load based on experimental data stored in the washing machine of before-water supply motor load and after-water-supply motor load with respect to a specific amount of laundry, and the data resetting step comprises resetting the control data based on the before-water-supply motor load estimated at the secondary laundry amount determination step when the before-water-supply motor load measured at the primary laundry amount determination step is different from the before-water-supply motor load estimated at the secondary laundry amount determination step. a primary laundry amount determination step (S10, S10', S100) of measuring the amount of the laundry placed in the drum before supplying wash water to the tub; a data setting step (S20, S20', S200) of setting control data to control the washing machine based on the amount of the laundry measured at the primary laundry amount determination step; a secondary laundry amount determination step (S50, S50', S500) of measuring the amount of the laundry placed in the drum after supplying wash water to the tub; a data resetting step (S65, S631', S631) of resetting the control data based on the amount of the laundry determined at the secondary laundry amount determination step when the amount of the laundry measured at the primary laundry amount determination step is different from the amount of the laundry measured at the secondary laundry amount determination step; a washing step (S70, S70', S700) of rotating the drum in a predetermined rotation direction, the washing step being performed after the data resetting step; characterized in that, the secondary laundry amount determining step includes an observation motion step comprising rotating the drum to a reference angle in a direction opposite to the rotation direction set at the washing step to measure electric current supplied to the motor, the observation motion step being performed before the washing step, wherein the primary laundry amount determination step comprises rotating the drum to a predetermined reference angle to measure before-water-supply motor load, the secondary laundry amount determination step further comprises a before-water-supply motor load estimation step comprising estimating before-water-supply motor load from the measured after-water-supply motor load based on experimental data stored in the washing machine of before-water supply motor load and after-water-supply motor load with respect to a specific amount of laundry, and the data resetting step comprises resetting the control data based on the before-water-supply motor load estimated at the secondary laundry amount determination step when the before-water-supply motor load measured at the primary laundry amount determination step is different from the before-water-supply motor load estimated at the secondary laundry amount determination step.