To provide an inertia device that maintains high accuracy in spite of variation in the period of an external clock signal, and outputs inertia data in synchronization with the external clock signal.

External clock period data 106 showing the period of the external clock signal 105 is supplied from a high-order external device 6 through a data bus 7 and an external connecting circuit 5 to an arithmetic processing part 4. The external clock period data 106 is obtained by measuring the repeat cycle of the external clock signal 105 in the high-order external device 6. Although the inertia operation conducted by the arithmetic processing part 4 includes operation for time integration, as time data in integral operation, the data on repeat time indicated by the external clock period data 106 is used, so the inertia data (attitude, speed and position) is high-accuracy data from which an influence of variation of repeat cycle Δt in the external clock signal 105 is eliminated.