A method for measuring the activity of creatine kinase MB isozyme which is accurate, high in specificity, convenient, by inhibiting the activity of a mitochondria localized creatine kinase isozyme to avoid the influence of mCK and a measurement reagent therefor. It is preferable that each of the plurality of groups has a priority different from the priorities of the others, and in arbitrating the frequency of each bus master owning the bus, arbitration of ownership of the bus by bus masters belonging to the same group is performed according to a round-robin method. The invention permits electronic devices to be made that are smaller and lighter, while improving their reliability.