An electronic control unit includes a receiver that receives a data frame transmitted at given transmission periods from a transmission source electronic control unit connected via a communication network, a buffer capable of storing the data frame, a writer that writes the data frame received by the receiver into the buffer, and an abnormality determiner that determines that the data frame is abnormal when the number of data frames written into the buffer exceeds a given threshold or when the data frame is written in excess of a capacity of the buffer.