PURPOSE: To facilitate assembly and disassembly and reduce resistance and wear during sliding.

CONSTITUTION: An internal corner form 26 intended to form internal corners of a wail structure by molding concrete has sliding plates 28 and 30 to form the corners. In addition, the form has an actuator 106 to expand and contract the form 26 and a pentagonal molding lid. A slide lug 72' that engages with the sliding plates and maintain the sliding plates in a correction direction is provided. The slide lug allows the relative sliding of the sliding plates, when the form expands and contracts. A lubricant is injected between the sliding plates, and smoothens the sliding of the plates. The lubricant forms a barrier that prevents a grout from exuding between form members and the lid.