The sound velocity measurement of longitudinal waves is of interest for nondestructively evaluating concrete structures because the velocity is closely related to the elastic properties of concrete and the standards of the method for measuring sound velocity in concrete have already been established in several countries. The descriptions of the measuring method are certainly correct, however, it may be difficult to obtain the same results from different instruments in practice not only because of the positioning method of the transmission and reception pulses but also materials, geometric forms and dimensions of the reference blocks for the instrument calibration are different for each kind of instrument. In order to obtain the same results from different instrument, it will be very significant to use a specified reference test block for calibrating each instrument. In accordance with these considerations, the reference block made with a core block of acrylic resin separating aluminium alloy plates was developed, which has the acoustical characteristics )acoustic impedance of test surface and sound velocity of longitudinal wave( nearly comparable to that of concrete. The effect of this reference block in actual measurement of the sound velocity in concrete was assessed for the drafting committee of JSNDI )Japanese Society for Non-Destructive Inspection(.