Weapon focus refers to a phenomenon that occurs when a crime witness focuses on the weapon, and as a result, experiences a reduced ability to remember the other details of the crime. The purpose of this study was to examine "unusualness" as a possible explanation for the weapon focus effect. In this experiment, this phenomenon was examined by playing videotapes that depicted an event in a classroom or a kitchen. In the videotapes, a male target held a knife while nearing a female person (control). It is considered usual for a person to hold a knife in the kitchen (usual condition), but it is unusual for a person to hold a knife in the classroom (unusual condition). In the test session, participants attempt to describe the male target and the control person's features. Results showed that the viewer's memory of the male target was poorer than their memory of the control person only when they viewed the event in an unusual condition. This result indicates that the weapon focus effect is caused by the usualness of the weapon.