Detecting idioms in a sentence is important to sentence understanding. This paper discusses the linguistic knowledge for idiom detection. The challenges are that idioms can be ambiguous between literal and idiomatic meanings, and that they can be “transformed” when expressed in a sentence. However, there has been little research on Japanese idiom detection with its ambiguity and transformations taken into account. We propose a set of linguistic knowledge for idiom detection that is implemented in an idiom dictionary. We evaluated the linguistic knowledge by measuring the performance of an idiom detector that exploits the dictionary. As a result, more than 90% of the idioms are detected with 90% accuracy.