According to the present invention, there is provided an artificial tendon-forming auxiliary instrument in which a valve cusp of a cardiac valve and a papillary muscle can be joined together with an artificial tendon through an easy procedure and the artificial tendon can be finished so as to have a desired length. According to an aspect of the present invention, an auxiliary instrument (100) is used for joining a valve cusp (210) of the cardiac valve and a papillary muscle (212) with an artificial tendon (200). A pair of end-portion support portions (20) and (21) respectively supporting the valve cusp (210) and the papillary muscle (212) is provided at both ends in a longitudinal direction. The auxiliary instrument (100) includes first and second tendon insertion ports through which the artificial tendon (200) is inserted, a tendon guide portion (30), and an instrument holding portion (40). The first and second tendon insertion ports are respectively formed in the pair of end-portion support portions (20) and (21) and have diameters greater than a diameter of the artificial tendon (200). The tendon guide portion (30) is formed so as to extend in the longitudinal direction across the first and second tendon insertion ports. The instrument holding portion (40) is formed in an intermediate portion of the tendon guide portion (30) in the longitudinal direction.