1. A motion detection apparatus comprising: an inertia motion detecting section which detects an inertial motion of an object to be measured by using at least one of an acceleration sensor and an angular velocity sensor; an image pickup section, which is fixed to the inertia motion detecting section, and which comprises a fish-eye lens for picking up a peripheral image of the object to be measured; an image comparing section which compares an image picked up at a first timing with an image picked up at a second timing by the image pickup section; a camera position and posture acquiring section which detects a position and a posture of the object to be measured by using a result obtained by the image comparing section; and a spatial position and posture acquiring section which detects a position and a posture in space of the object to be measured based on outputs from the inertia motion detecting section and the camera position and posture acquiring section, wherein when the camera position and posture acquiring section detects the position and posture of the object to be measured, the camera position and posture acquiring section sets a coordinate system for motion detection, extracts a feature point based on the image at the first timing picked up by the image pickup section, sets a rectangular area having the feature point as a center of position, registers a position and a posture of the image pickup section in the coordinate system, calculates a position and a posture of the image pickup section at the second timing based on relative motion information obtained by the inertia motion detecting section and the position and posture of the image pickup section as registered at the first timing, estimates and stores coordinates of the image at the second timing, in the rectangular area, by calculating an estimated position of each pixel in the rectangular area based on the position and posture information and a relational expression of an image height on an image forming surface at an angle of incidence on the fisheye lens, and executes a matching process based on the rectangular area registered at the first timing and the rectangular area stored at the second timing.