There are provided a processor device for an endoscope capable of accurately acquiring an observation distance, an operation method thereof, and a control program. An endoscope system (10) has an endoscope (12), and a light source device (14), and a processor device (16). The endoscope (12) has an imaging sensor (46), and images an observation target and outputs an image signal. The light source device (14) emits illumination light for illuminating the observation target. The processor device (16) includes a measurement image processing unit (64). The measurement image processing unit (64) includes a blood vessel index value calculation section (72) and an observation distance calculation section (74). The blood vessel index value calculation section (72) calculates a blood vessel index value based on the image signal from the endoscope (12). The observation distance calculation section (74) calculates an observation distance from the blood vessel index value calculated by the blood vessel index value calculation section (72).
