The invention provides a reamer apparatus for a ground boring machine that is capable of restricting increase in drawing resistance when performing drawing of a buried pipe by using a reamer and of efficiently performing drawing operations of the buried pipe without degrading properties such as flexibility and rotatability. The reamer apparatus includes a reamer main body of substantially hollow conical shape that reduces in diameter towards a drawing side, and a rod connecting portion that provided at a narrow diameter end portion of the reamer main body and connected with a rod. The reamer main body is connected to a buried pipe through a coupling structure disposed on an opposite side to the rod connecting portion. The coupling structure has a Swivel joint that allows rotation of the reamer main body with respect to the buried pipe, wherein a main portion of the Swivel joint is substantially accumulated in the reamer main body.