1. A method of examining a vascular endothelium function by an automatic measurement system comprising: a measuring system comprising an ultrasound diagnostic device and an ultrasound probe attached to said ultrasound diagnostic device for acquiring an ultrasound image; an image processing and calculating system comprising a computer and an image processing board for processing the acquired ultrasound image by said measuring system; and a navigation system comprising a robot arm for moving and rotating said ultrasound probe, wherein said method comprises steps of: a first step wherein an ultrasound image is acquired after said ultrasound probe is moved to a subject portion for probing its vascular endothelium function and said acquired image is processed for calculating and controlling a position of said ultrasound probe so that the center of a blood vessel to be probed is laid on the center of said image; a second step wherein a series of operations comprising an operation for acquiring ultrasound image, an operation for processing and calculating the acquired ultrasound image and an operation for moving said robot arm based on the calculated result, are repeated starting from the finally acquired ultrasound image at said first step so that said ultrasound probe is moved to in a parallel position to the center line of the blood vessel; a third step wherein a series of operations comprising an operation for acquiring ultrasound image, an operation for processing and calculating the acquired ultrasound image and an operation for moving said robot arm based on the calculated result, are repeated starting from the finally acquired ultrasound image at said second step so that said ultrasound probe is moved to a position where said ultrasound probe is in parallel to and immediately above the center line of the blood vessel; and a fourth step wherein a blood vessel diameter is determined from the finally acquired ultrasound image at said third step.