A communication device is connected to a plurality of external communication devices. The communication device includes one or more hardware processors. The processors receive frames transmitted from the plurality of external communication devices at timings prescribed for each of the plurality of external communication devices. The processors calculate, for each of the plurality of external communication devices, an arrival ratio indicating a ratio at which transmitted frames have reached the communication device, based on a number of timings in a measurement period and a number of frames received in the measurement period.