Use of an air conditioner being installed indoors, the air conditioner comprising an infrared sensor, wherein the infrared sensor includes a multi-element light-receiving unit formed of a plurality of light-receiving elements (2a-2h) aligned on a straight line, and a condenser lens; and wherein the multi-element light-receiving unit is disposed at a position of receiving infrared rays which have passed through the condenser lens, such that the plurality of light-receiving elements (2a-2h) are aligned in a vertical direction with respect to an indoor floor surface, and such that an intersection between the multi-element light-receiving unit and a perpendicular line from a focal point of the condenser lens to the multi-element light-receiving unit is deviated from a center of the multi-element light receiving unit, wherein the perpendicular line coincides with an optical axis of the condenser lens, wherein the multi-element light-receiving unit is disposed such that the intersection between the multi-element light-receiving unit and the perpendicular line from the focal point of the condenser lens to the multi-element light-receiving unit is positioned in a lower half of a whole length of the multi-element light-receiving unit in the vertical direction, wherein the multi-element light-receiving unit is disposed such that the intersection between the multi-element light-receiving unit and the perpendicular line from the focal point of the condenser lens to the multi-element light-receiving unit is positioned in a range of (N - m) ÷ N to (N - n) ÷ N of a whole length of the multi-element light-receiving unit in the vertical direction, where N is the number of the light-receiving elements (2a-2h) forming the multi-element light-receiving unit; m is an integer obtained by rounding off a value of computation of m = (tan -1 (D ÷ (h -L)) - (90 -δ - N × θ ÷ 2)) ÷ θ to a nearest whole number; n is an integer obtained by rounding off a value of computation of n = (tan -1 (D ÷ h) - (90 - δ - N × θ ÷ 2)) ÷ θ to a nearest whole number; D is a horizontal distance between the infrared sensor and a target of which heat detection is performed by the infrared sensor; h is an installation height of the infrared sensor; L is a height of the target of which heat detection is performed by the infrared sensor; δ is a depression angle of the infrared sensor, the depression angle being an installation angle of the infrared sensor with respect to a horizontal line; and θ is a view angle (11a-11h) of each light-receiving element (2a-2h), the view angle (11a-11h) being an angle in the vertical direction at which each light-receiving element (2a-2h) can detect infrared ray. the multi-element light-receiving unit is disposed such that the intersection between the multi-element light-receiving unit and the perpendicular line from the focal point of the condenser lens to the multi-element light-receiving unit is positioned in a range of (N - m) ÷ N to (N - n) ÷ N of a whole length of the multi-element light-receiving unit in the vertical direction, where N is the number of the light-receiving elements (2a-2h) forming the multi-element light-receiving unit; m is an integer obtained by rounding off a value of computation of m = (tan -1 (D ÷ (h -L)) - (90 -δ - N × θ ÷ 2)) ÷ θ to a nearest whole number; n is an integer obtained by rounding off a value of computation of n = (tan -1 (D ÷ h) - (90 - δ - N × θ ÷ 2)) ÷ θ to a nearest whole number; D is a horizontal distance between the infrared sensor and a target of which heat detection is performed by the infrared sensor; h is an installation height of the infrared sensor; L is a height of the target of which heat detection is performed by the infrared sensor; δ is a depression angle of the infrared sensor, the depression angle being an installation angle of the infrared sensor with respect to a horizontal line; and θ is a view angle (11a-11h) of each light-receiving element (2a-2h), the view angle (11a-11h) being an angle in the vertical direction at which each light-receiving element (2a-2h) can detect infrared ray.