Reading Japanese historical manuscripts is one of the first steps for researching Japanese classical literature. It is, however, difficult and time-consuming even for Japanese people to read such manuscripts since they are handwritten in cursive style and may contain different characters from those currently used. We formulated the human process for reading Japanese historical manuscripts as a constraint satisfaction problem, and proposed a framework to assist the process. In this paper, we present a constraint solving Web service as a part of a system based on the framework. To realize the Web service, we added a Web service layer to our constraint solver previously implemented in Ruby as a UNIX command. Thanks to the loose coupling realized by the Web service, any programming language can be used for implementation of other parts of the whole system. We experimentally confirmed the solver as a Web service is faster than that as a UNIX command if both the solver and a client are connected to a same local area network. We finally summarized technical issues concerning the system based on the framework.