Explicit formula and efficient computing algorithm for the least-cost insertion string in the LR-based error repair scheme are presented. The formula for the least-cost insertion string is derived from the expected vocabulary string which is an extension of LALR lookahead sets. Based on a new LALR formalism[PCC85], an efficient algorithm for the expected vocabulary string is devised. This scheme can be viewed as an improvement of Fischer, et al.``s scheme[FDM79]. In order to compare with their method, we have implemented an error repair table constructor by modifying their parser generator(ECP). Experimental results including comparison are presented with respect to time and space characteristics in construction of the error repair table and computation of the locally least-cost repair in LR-based parsers.