1. An ultrasonic diagnosis apparatus comprising: an ultrasonic probe configured to transmit ultrasonic waves to a subject and receive ultrasonic waves from the subject; a computer including at least one processor, performing operations including: reconstructing an ultrasonic image captured in real time, on a basis of a reflection echo signal measured by the ultrasonic probe; holding volume data of the subject captured in advance, in a non-transitory storage medium; extracting, from the volume data held in the storage medium, a reference image corresponding to a sectional position of the ultrasonic image captured in real time; and displaying a reconstructed said ultrasonic image captured in real time and the reference image, wherein: the holding holds time-series volume data of plural shifted slice-planes of the subject captured in advance, in which time phase information are attributed to two-dimensional image data of respective time-series cross-sectional images taken in each slice-plane, and the two-dimensional image data of the cross-sectional images of the slice planes, are sorted via the time phase information into two-dimensional image data groups, with each respective two-dimensional image data group including the two-dimensional image data having time-phase information of a same time phase; and the extracting acquires, from the storage medium, volume data composed of a two-dimensional image data group with time-phase information which coincides with the time-phase information of the ultrasonic image captured in real time, and extracts and reconstructs the reference image from volume data which has a slice plane which coincides to a slice plane of the ultrasonic image captured in real time; the time-phase information and a type of a biological signal indicated as an input to the ultrasonic diagnosis apparatus and used to create the time-phase information, are added to the two-dimensional image data of respective cross sections of volume data held in the storage medium; and when a reference-image display function is started, the ultrasonic diagnosis apparatus checks whether or not the type of a biological signal fed to an input of the ultrasonic diagnosis apparatus in connection with real-time said reflection echo signal measured by an ultrasonic probe, coincides with the type of the biological signal added to the two-dimensional image data of respective cross-sections of the volume data held in the storage medium, and provides a warning when coincidence is not found.