A detection area 112 is set in a three-dimensional space in which a subject exists. When an actual hand enters the detection area 112, coordinate points (white and black dots) represented by pixels making up a silhouette 114 of the hand in a depth image enter the detection area 112. In the detection area 112, a reference vector 126 is set that shows the direction which the hand should face relative to the shoulder as a reference point 122. Then, an inner product between two vectors, a vector from the reference point 122 to each of coordinate points and the reference vector 126, is calculated, followed by comparison between the inner products. Positions of coordinate points whose inner products are ranked high are acquired as the position of tips of the hand.