An air conditioner may include: a motion detection sensor configured to detect a movement of an object; at least one processor; a memory comprising instructions configured to cause the at least one processor to: determine, based on the motion detection sensor detecting the object, an activity amount of the object via the motion detection sensor, determine, based on the activity amount of the object satisfying a predetermined first condition, whether a number of second predetermined time sections, in a first predetermined time section, in which the object is detected via the motion detection sensor satisfies a predetermined second condition, or whether the activity amount of the object detected via the motion detection sensor in the first predetermined time section satisfies a predetermined third condition, and determine, based on determining that the predetermined second condition or the predetermined third condition is satisfied, that the object exists.