A first region of interest and a second region of interest are set respectively including a first end and a second end of a blood vessel of interest running in an umbilical cord. A template representing a fluctuation of a pulse at the first end is produced from a change of brightness information in the first region of interest. A graph representing a fluctuation of a pulse at the second end is produced from a change of brightness information in the second region of interest. A propagation time of blood is calculated by identifying a portion, in the graph, which matches the template. A length of the umbilical cord is estimated based on the propagation time.