Monitoring device

A monitoring device conducts a process of obtaining pieces of distance data representing distances to a plurality of physical objects present in a monitoring region, from measurement results of the monitoring region, and using the distance data as current distance data; obtaining past distance data from the measurement results, and converting the past distance data into comparison distance data; calculating difference values between the current distance data and the comparison distance data, and extracting changed regions whose difference values are greater than or equal to a threshold; creating an image obtained by transforming a frontal viewpoint image such that a viewpoint of the measurement of the monitoring region is moved, the frontal viewpoint image being based on the current distance data and the changed regions; and identifying the plurality of physical objects present in the monitoring region, on the basis of the frontal viewpoint image and the coordinate transformed image.