To provide a rod chuck in a penetration testing machine by which an operator can install a penetration rod quickly and accurately.

A rod chuck in a penetration testing machine includes a penetration rod 4 having engaging grooves on the side face, a hollow chuck shaft 11 having an internal hole in which the penetration rod can be inserted; a sleeve that is capable of reciprocating along the chuck shaft 11 and continuously energized on the chuck shaft 11's tip side; a sleeve presser 14 that controls movement of the sleeve to the chuck shaft 11's tip side and in which the top face is struck if necessary; and steel balls 17a, 17b, and 17c that are disposed in the chuck shaft 11 and continuously supported by the sleeve in a state of projecting in the inner hole of the chuck shaft 11 and fitted to the engaging grooves in the penetration rod 4 to hold the penetration rod 4. Marks 18a, 18b, and 18c that indicate the arrangement positions of the steel balls 17a, 17b, and 17c are installed in the tip face of the chuck shaft 11.