In a monitoring apparatus, an optical sensor senses objects present in a sensing area, and moving speeds and moving directions of the objects are calculated based on changes with time in positional information of the sensed objects. An object which meets a predetermined moving condition is determined as a moving object. Hence an entrance of the moving object into the sensing area can be monitored. In this apparatus, by a determining section, it is determined whether or not an object is sensed in an area (after-passage sensing area) which is set to include positions adjacent to positions through which the sensed moving object has passed. When the object has been sensed in the area, by the determining section, it is further determined whether or not the object has remained for a predetermined determination period of time or more. This determined result is outputted by an outputting section.