An apparatus and method for automatic translation of patent documents are provided. Translation knowledge customized for the patent domain is extracted and used. The method includes the steps of: extracting a number of technical terms from a patent document and assigning translated terms for the extracted technical terms; extracting expressions frequently used in patent documents, not in a document in a general domain, and constructing translated expressions; using the translation knowledge constructed in this way, applying a sentence pattern to extraordinarily long sentences in a patent document, dividing the sentence by recognition of parallel structure and syntax clues; and extracting translation units available for parsing and performing structure analysis.