1. A server apparatus that generates an HTML file upon receipt of an HTTP request, the server comprising: an application executor device that generates the HTML file based on the HTTP request; and an HTTP server device that receives the HTTP request and sends an HTTP response to the HTTP request, the HTTP response including the HTML file generated by the application executor device, wherein: the application executor device includes means for executing a script in generating the HTML file, for calculating the number of appearances of the string when an instruction code in the script is a command for processing a string, and for storing the string in a storage as a string file when the use frequency score of the string is higher than a preset value, the application executor device further includes means for executing a script in generating the HTML file and replacing the string with reference information for reference to the string file when (i) an instruction code of the script is a command for processing a string and (ii) when the string matches the stored string, and the HTTP server includes means for sending the string file indicated by the reference information along with the HTML file when the HTML file generated by the application executor includes the reference information for reference to the string file; wherein the use frequency score of the string is a function value based on: the length of a string assigned to an argument of an instruction code; time needed for memory copy; time needed for a system call; time needed for string comparison performed in execution of the instruction code; and the number of appearances of the string.