A washing machine which comprises: a tub; a drum rotatably installed in the tub; a water supply portion configured to supply water to the tub; a driving portion configured to rotate the drum; at least one processor configured to control the water supply portion and the driving portion to perform washing operations including a water supply operation, a drainage operation, a spin-drying operation; and a water level detector configured to detect a water level in the tub, characterised in that: the at least one processor is configured to: calculate first and second water level change rates which indicate water level change amounts per a first period of time and a second period of time respectively based on a detection result of the water level detector during the water supply operation or the drainage operation; calculate a ratio of the second water level change rate to the first water level change rate; determine that a prelude to abnormal vibrations is present when the calculated ratio is greater than a preset threshold value, and based on determining that the prelude to abnormal vibrations is present, the at least one processor controls an operation of the driving portion to rotate the drum at or below a certain rotation number during the spin-drying operation. a tub; a drum rotatably installed in the tub; a water supply portion configured to supply water to the tub; a driving portion configured to rotate the drum; at least one processor configured to control the water supply portion and the driving portion to perform washing operations including a water supply operation, a drainage operation, a spin-drying operation; and a water level detector configured to detect a water level in the tub, characterised in that: the at least one processor is configured to: calculate first and second water level change rates which indicate water level change amounts per a first period of time and a second period of time respectively based on a detection result of the water level detector during the water supply operation or the drainage operation; calculate a ratio of the second water level change rate to the first water level change rate; determine that a prelude to abnormal vibrations is present when the calculated ratio is greater than a preset threshold value, and based on determining that the prelude to abnormal vibrations is present, the at least one processor controls an operation of the driving portion to rotate the drum at or below a certain rotation number during the spin-drying operation. the at least one processor is configured to: calculate first and second water level change rates which indicate water level change amounts per a first period of time and a second period of time respectively based on a detection result of the water level detector during the water supply operation or the drainage operation; calculate a ratio of the second water level change rate to the first water level change rate; determine that a prelude to abnormal vibrations is present when the calculated ratio is greater than a preset threshold value, and calculate first and second water level change rates which indicate water level change amounts per a first period of time and a second period of time respectively based on a detection result of the water level detector during the water supply operation or the drainage operation; calculate a ratio of the second water level change rate to the first water level change rate; determine that a prelude to abnormal vibrations is present when the calculated ratio is greater than a preset threshold value, and based on determining that the prelude to abnormal vibrations is present, the at least one processor controls an operation of the driving portion to rotate the drum at or below a certain rotation number during the spin-drying operation.