A method for operating a clothes treating apparatus comprising: a drum configured to accommodate therein an object to be dried; an air suction means configured to form a flow path of air introduced into the drum; an air exhaustion means configured to form a flow path of air exhausted from the drum; a condenser disposed to heat air sucked into the drum through the air suction means; an evaporator disposed to cool air exhausted from the drum through the air exhaustion means; a compressor configured to compress a refrigerant introduced from the evaporator; a variable expander configured to expand a refrigerant introduced from the condenser, and having a variable open degree; a humidity sensor configured to measure a humidity of air exhausted from the drum; and a controller configured to change an open degree of the variable expander; the method comprising: measuring the humidity of exhaust air exhausted from the drum by the humidity sensor; and changing, by the controller, the open degree of the variable expander according to the measured humidity of the exhaust air, and characterized in that in the step of changing the open degree of the variable expander, the variable open degree is decreasingly set as the measured humidity of the exhaust air decreases, and if the variable open degree of the variable expander is to be decreased, an open degree decrease width of the variable open degree is set to be narrower when the detected humidity is low than when the detected humidity is high. a drum configured to accommodate therein an object to be dried; an air suction means configured to form a flow path of air introduced into the drum; an air exhaustion means configured to form a flow path of air exhausted from the drum; a condenser disposed to heat air sucked into the drum through the air suction means; an evaporator disposed to cool air exhausted from the drum through the air exhaustion means; a compressor configured to compress a refrigerant introduced from the evaporator; a variable expander configured to expand a refrigerant introduced from the condenser, and having a variable open degree; a humidity sensor configured to measure a humidity of air exhausted from the drum; and a controller configured to change an open degree of the variable expander; the method comprising: measuring the humidity of exhaust air exhausted from the drum by the humidity sensor; and changing, by the controller, the open degree of the variable expander according to the measured humidity of the exhaust air, and characterized in that in the step of changing the open degree of the variable expander, the variable open degree is decreasingly set as the measured humidity of the exhaust air decreases, and if the variable open degree of the variable expander is to be decreased, an open degree decrease width of the variable open degree is set to be narrower when the detected humidity is low than when the detected humidity is high. measuring the humidity of exhaust air exhausted from the drum by the humidity sensor; and changing, by the controller, the open degree of the variable expander according to the measured humidity of the exhaust air, and characterized in that in the step of changing the open degree of the variable expander, the variable open degree is decreasingly set as the measured humidity of the exhaust air decreases, and if the variable open degree of the variable expander is to be decreased, an open degree decrease width of the variable open degree is set to be narrower when the detected humidity is low than when the detected humidity is high.