Abstract: A blood vessel diameter measuring apparatus includes an ultrasonic probe which emits an ultrasonic wave toward a blood vessel, receives an ultrasonic wave reflected from the blood vessel and outputs an ultrasonic signal based on the received ultrasonic wave, a detecting section which is attached to the ultrasonic probe and outputs information indicating a position of the ultrasonic probe and an attitude of the ultrasonic probe with respect to the blood vessel, a support fixture which supports the ultrasonic probe so as to enable the ultrasonic probe to perform at least one of operations, an image processing section which produces an ultrasonic image of the blood vessel by using the ultrasonic signal from the ultrasonic probe and the position/attitude information from the detecting section, and a measuring section which measures a blood vessel diameter of the blood vessel by using the produced ultrasonic image.