A random number generating device includes: a microcomputer; a first oscillator circuit that has a predetermined temperature characteristic and generates a clock serving as a basis of a behavior of the microcomputer; and an electronic circuit that has a temperature characteristic being different from the predetermined temperature characteristic of the first oscillator circuit and operates in accordance with a command from the microcomputer. The microcomputer measures an operating time of the electronic circuit based upon the clock generated by the first oscillator circuit and generates a random number based upon a result of the measurement.