PURPOSE: To obtain a strain gauge which can be easily and safely fitted to a pile even by an unskilled person, can be repeatedly used for another pile by easily removing the gauge from the pile after measurement is completed, and can simultaneously measure a strain and acceleration with high responsiveness and sensitivity.

CONSTITUTION: In a strain starting body 1, a constricted part 4 is formed by axisymmetrically forming two elliptical openings 5 and 5' with respect to a reference line 1a and slits 6 and 6' are respectively formed from the openings 5 and 5' to both sides of the body 1. Both-end-supported beam type strain starting sections 10 and 10' are formed by forming a lateral rectangular opening 9 below the openings 5 and 5' and section 4 and four strain gauges 12a and 12b are stuck to the lower surface 9a of the opening 9. At the time of fitting the body 1 to a steel pile, two bolts 18 are inserted into holes 2 and 3 while a protective plate 17 is fitted with small bolts 19 and the bolts 18 are tightened in female screw sections formed in the pile.