PURPOSE: To perform automatic and accurate measurement of an N value, in a standard penetration test for earth.

CONSTITUTION: A weight upper limit stop head T having a limit switch R for demagnetization and a knocking head N are mounted on a guide rod G for a weight W for a boring machine A and a measurement rod K in which measurement spots K1 and K2 are engraved is relayed thereto. Anchoring to the upper end of a sleeve U of a boring machine is effected, and a transparent cylinder type sensor head S having a sensor spot S1 is fitted therein. When the sensor spot S and the measurement spot attain a detecting distance, the electromagnetic clutch brake unit of the bailing boring is excited resultant from reaction to a sensor. After the weight is raised to the head T along a weight guide rod, the weight effects free drop and strikes the rod on the knocking head N. Hammering is effected at the same pitch until a boring rod L is penetrated to a specified penetration amount and the number of hammering times during a time in which the rod is penetrated to the specified penetration amount is displayed.