PURPOSE: To positively confirm it externally out of an excavated groove whether or not the entire surface of a presser plate is completely in contact with the ground by providing a plurality of detection members capable of advancing/retreating, which detect that each presser plate is brought in contact with the excavated ground, at the plural places of the presser plate.

CONSTITUTION: Attachments 7 detecting whether or not a presser plate 1 is completely in contact with the excavated ground are mounted onto the four corners of the presser plate 1 to which each earth pressure meter is mounted. A jack cylinder 3 is installed in a reinforcement cage so as to be inserted in an excavated groove, and the cylinder is set up so that the presser plate 1 is directed to the ground at the circumference of the excavated groove. When the presser plate 1 is advanced against the ground with the cylinder 3 operated, if the surface of the ground is flat, all of the contact plates 17 of the attachments 7 retreat against coil springs 21, and the entire surface of the presser plate 1 is brought in contact with the ground. At this moment, since a lead switch 13 is turned on by a magnet 19 so as to allow all of four LEDs on the earth to be lighted, it is thereby affirmed that the presser plate 1 is normally in contact with the ground.