A basic information calculator device calculates basic information for area limiting excavation control to control a construction machinery work device so that the construction machinery does not perform excavation beyond a target excavation surface. The basic information calculator includes: a storage device having stored therein three-dimensional information of the target excavation surface; a two-dimensional information extractor for obtaining the intersecting line between a reference surface that is the target excavation surface and the operational plane of the work device based on the three-dimensional information and the current positional information of the construction machinery to extract a reference line that is the intersecting line as the two-dimensional information of the reference surface in the operational plane; and a characteristic point transmitter for transmitting the Z-coordinates of characteristic points on the reference line to an area limiting excavation controller as the basic information.