A random number generating device includes a processing request part, a receiving part, a counting part, and a random number generation part. The processing request part makes a request for a predetermined processing to a processing requested object. The receiving part receives a response from the processing requested object in response to the request. The counting part performs a counting operation to increase or decrease a count value with a predetermined cycle, the counting operation being started at least prior to receipt of the response. The random number generation part acquires at least one count value of the counting part at least one predetermined acquisition timing after the receipt of the response is started, and generates a random number by using the at least one count value that is acquired.