A position measuring device comprising: an image acquisition section comprising a photographing device for photographing and acquiring a series of photographed images changing continuously or changing little by little along with position displacement relative to an object to be photographed; a positional relationship measurement section for measuring position displacement and posture change of the image acquisition section; a feature extraction section for extracting feature points from the photographed images obtained with the image acquisition section; a feature point tracking section for tracking the feature points by searching corresponding points corresponding to the feature points on a plurality of the photographed images; characterised by: an image-and-feature point selection section for selecting feature points and images suitable for 3D measurement to be processed from the series of photographed images based on a measurement result made with the positional relationship measurement section, and for determining a pair of stereo images having sufficient baseline length from the images to be processed for position measurement; and a position measurement section for measuring the photographing position and photographing posture of the image acquisition section or position of the photographed object based on the feature points of the pair of stereo images determined with the image-and-feature point selection section, the feature points being tracked with the feature point tracking section. an image acquisition section comprising a photographing device for photographing and acquiring a series of photographed images changing continuously or changing little by little along with position displacement relative to an object to be photographed; a positional relationship measurement section for measuring position displacement and posture change of the image acquisition section; a feature extraction section for extracting feature points from the photographed images obtained with the image acquisition section; a feature point tracking section for tracking the feature points by searching corresponding points corresponding to the feature points on a plurality of the photographed images; characterised by: an image-and-feature point selection section for selecting feature points and images suitable for 3D measurement to be processed from the series of photographed images based on a measurement result made with the positional relationship measurement section, and for determining a pair of stereo images having sufficient baseline length from the images to be processed for position measurement; and a position measurement section for measuring the photographing position and photographing posture of the image acquisition section or position of the photographed object based on the feature points of the pair of stereo images determined with the image-and-feature point selection section, the feature points being tracked with the feature point tracking section. an image-and-feature point selection section for selecting feature points and images suitable for 3D measurement to be processed from the series of photographed images based on a measurement result made with the positional relationship measurement section, and for determining a pair of stereo images having sufficient baseline length from the images to be processed for position measurement; and a position measurement section for measuring the photographing position and photographing posture of the image acquisition section or position of the photographed object based on the feature points of the pair of stereo images determined with the image-and-feature point selection section, the feature points being tracked with the feature point tracking section.