Vehicle-mounted communication system, switch device, abnormality detecting method, and abnormality detecting program

A non-transitory computer-readable storage medium having, stored therein, a abnormality detection program used in a switch device in an in-vehicle communication system provided with an in-vehicle device group including four or more in-vehicle devices including the switch device,

• the program causing a computer to function as:
• a relay unit configured to relay a packet between the in-vehicle devices;
• and
• a detection unit configured to detect abnormality of a specific in-vehicle device, other than the switch device, in the specific in-vehicle device group, based on a reception state of a specific target packet that circulates through a specific in-vehicle device group that is a part of the in-vehicle device group and includes three or more specific in-vehicle devices being the in-vehicle devices.