Provided are an automatic translation device and a method thereof. The automatic translation device includes: a character string error correcting unit correcting an error of an input text; a language simplification processing unit substituting the input text with a simplified input text by applying context based constraints and rules corresponding thereto; and an automatic translation unit translating the simplified input text into an output text. As a result, it is possible to ultimately improve automatic translation quality by mitigating inconvenience of an input text writer due to restriction of a vocabulary using list and a text configuring rule.