ISO 26262 (Road vehicles - Functional safety), a functional safety standard for motor vehicles, was published in November 2011. In this standard, hazardous events associated with each item constituting a safety-related system are assessed according to three criteria, namely, Severity, Exposure, and Controllability, thereby determining ASILs (Automotive Safety Integrity Levels) representing safety levels for motor vehicles. Although motorcycles are not included in the scope of application of the current edition of ISO 26262, it is expected that motorcycles will be included in the next revision. However, it is not appropriate to directly apply ASILs to motorcycles. In the first place, the situation of usage in practice presumably differs between motorcycles and motor vehicles. Accordingly, in this research, we attempted to newly define Motorcycle Safety Integrity Levels (MSILs). We demonstrate in this article that it is to reduce the maximum severity in the Correspondence Diagram between Risk and ASIL (CDRA) and to increase the degree of acceptable risks in view of situations specific to motorcycles. This makes it to considerably reduce the unacceptable risk area in the CDRA compared with that for motor vehicles. We newly defined four MSILs in this risk area. As a result of investigating connections between MSILs and ASILs, it was revealed that the adequacy of safety measures required against risks associated with motorcycles was less compared with the case of motor vehicles.