A defrosting determination device includes: a condition specifying unit that specifies the air conditioning load condition corresponding to a pair of temperature inside the room and temperature outside the building based on condition information; a slope specifying unit that specifies the slope corresponding to the specified air conditioning load condition based on slope information; a frequency acquiring unit that acquires frequency information specifying the operation frequency of the motor; a calculation unit that calculates a threshold value by multiplying the operation frequency by the slope; a power information acquiring unit that acquires power information specifying power output to the motor; a decision unit that decides whether the power is equal to or less than the threshold value; and a determination unit that determines to start defrosting operation in response to the decision unit deciding that the power is equal to or less than the threshold value.