To provide a pile driver by which the effect of soil improvement can be simply confirmed in a short time while effectively avoiding reduction of the operation rate and cost-up.

This pile driver is provided with a transfer means, a pile-driving means, a leader 2, a head 4 vertically movable on the leader 2, and a boring rod 5 restrained rotatably in a posture parallel to the leader 2, having a length corresponding to the leader 2 and a bit 5a at the front end. Further the driver comprises a sampling rod 6 restrained vertically insertably in a posture parallel to the leader 2 at the head 4, having a length corresponding to the leader 2 and equipped with a sampler 6a at the front end, a torque imparting means (rotary head 7) for imparting a tortque to the boring rod 5, an excavating force imparting means (winch 8 for the boring rod) for imparting an excavating force to the boring rod 5 a slurry supply means 10 for supplying slurry to the boring rod 5, and a percussion imparting means (automatic dropping device 11) for imparting percussions for measuring N value using boring holes bored by the boring rod 5 to the sampling rod 6.