PURPOSE: To accurately measure the revolution number though revolution number measurement processing is carried out with low priority interruption handling in a revolution number measuring device used in an internal combustion engine.

CONSTITUTION: A signal 10 representing revolution of an engine is put both into an input-capture input terminal 11 and an input terminal 12 for external interruption handling with lower interruption priority order, and revolution number is measured by referring to a value of a free running counter 14 latched by a register 13 in the input-capture input in the interruption handling with low interruption priority order. Thus, the accurate revolution number can be obtained regardless of processing of low level interruption.