Abstract: Parser generators such as YACC have been used in a large number of applications by non-specialized developers, not just those that involve compiler construction. A consequence of this is that good support is required for the comprehension of LALR(1) parsing techniques in order to developing correct, complete and conflict-free parsing grammars. Several types of potential problems in a grammar input to LALR(1) parser generators are defined, and an LALR(1) parser visualization and debugging system called VPGE is described. VPGE is an interactive system visualizing operations of the parser, supporting step by step simulation of the generated parser as well as breakpoints attached to grammar productions. Experiment result shows that the speed of parser generation in VPGE is even faster than that in GNU's Bison, which makes VPGE a fast LALR(1) grammar debugging environment.