This study purports to clarify the nature of hiniku (a quasi-equivalent of irony in the Japanese language). First, after reviewing Utsumi's (2000) implicit display theory of iron, I will propose the concept of ‘communicative insincerity’: verbal or non-verbal expressions made in a non-serious or distorted manner. Then I will present a framework explaining the communication of irony: an ironic remark is accompanied by communicative insincerity and alludes to the speaker's failed expectation; through this process, the speaker's negative emotion or evaluation is expressed (the communicative insincerity theory). I will analyze the use of hiniku based on 362 examples from novels, newspaper or magazine articles, etc. These examples are categorized into 12 groups: Reversals in Assertives, Reversals in Non-assertives, Infelicitous Questions, Unrealistic Assumptions, Juxtaposition of Two Events, Various Interpretation of Situations, Rhetorical Techniques, Inappropriate Style, Replacement, Echoing, Non-verbal Techniques, and No Insincerity. These groups can be hierarchically arranged. While the examples of hiniku are diversified in verbal and non-verbal features, most of them can be shown to be consistent with the above-mentioned theory. Finally, hiniku will be compared with English irony and sarcasm.