An image for an operator is extracted, and an operation determination unit employs a relationship, relative to a marker of an operation input system, for the operator, who is standing behind the marker when viewed by a video camera. When a body part of the operator comes to the front of an operation plane, as viewed by the video camera, the operation determination unit determines that an action for an input operation has been started, and examines the shape and the movement of each individual body part (an open hand, two fingers that are being held up, etc.), and determines whether the shape or the movement is correlated with one of operations that are assumed in advance.