This paper presents a text-based syntax completion method using an LR parser. We propose formal definitions of candidate text to be completed based on the sentential forms, and we design algorithms for computing candidates through reductions in the LR parsing. This is in contrast to the existing methods that have not clearly stated what candidates they intend to produce. This is also different from a transformation approach using an LR parser, which transforms the grammar of the programming language, a burdensome task at this moment. The advantage of our method is that LR parsers can be adopted without modification, and a syntax completion system can be built using them, without incurring efforts. We implemented the algorithms as an Emacs server to demonstrate the feasibility of their application.