1. A method for determining a convention violation of a program on a computer comprising a violation storing unit for beforehand storing violation information indicating a structure of the program violating a convention for each of a plurality of conventions; and a conversion rule storing unit for storing a predetermined plurality of conversion rules, comprising steps of: receiving an input of the program potentially violating the convention; selecting one or more conversion rules; sequentially applying the one or more of conversion rules into an abstract syntax tree of the program; converting the abstract syntax tree into a plurality of conversion trees; storing a pre-conversion sub-tree of the abstract syntax tree based on the abstract syntax tree prior to conversion into the plurality of conversion trees; storing a post-conversion sub-tree of the abstract syntax tree based on the converted abstract syntax tree; associating the pre-conversion sub-tree of the abstract syntax tree with the post-conversion sub-tree of the abstract syntax tree; determining that the program violates the convention on condition that at least part of a structure is common between the pre-conversion and post-conversion sub-trees, and that the common part of the structure satisfies any of the violation information stored in the violation storing unit; and informing a user of the program that the program violates the convention.