A jig for processing a cylinder block for being used in finish processing for a cylinder bore in the cylinder block, which comprises: a jig body, which has an opposed surface facing a cylinder head mounting surface of the cylinder block and which is provided so that the opposed surface and the cylinder head mounting surface relatively come close to and disengage from each other, with the opposed surface facing the cylinder head mounting surface; a bolt member, which is saliently provided onto the opposed surface as a portion having a rod-like outer shape that is capable of being inserted into a bolt hole for mounting the cylinder head opening on the cylinder head mounting surface, and which has multiple bolt pieces that are formed on the lateral surface thereof with a threaded engageable to a female threaded portion of the bolt hole, characterized in that it forms a tapered bore portion that is tapered on the opposed surface side and that is opened on the opposite side of the opposed surface side by the internal surfaces of the bolt pieces, so as to expand the diameter thereof by displacing the multiple bolt pieces due to a wedge action receiving through the tapered bore portion; a tapered shaft member, which has a tapered surface portion that is inserted into the tapered bore portion with the tapered surface portion projecting from the tapered bore portion and that is configured to provide the wedge action by the relative movement between the bolt member thereof in the insertion direction; and a piston member, which is provided so that it can be biased by a given suppress strength in the direction projecting toward the opposed surface, and which has a pressing surface contacting a prescribed surface portion of the cylinder head mounting surface. a jig body, which has an opposed surface facing a cylinder head mounting surface of the cylinder block and which is provided so that the opposed surface and the cylinder head mounting surface relatively come close to and disengage from each other, with the opposed surface facing the cylinder head mounting surface; a bolt member, which is saliently provided onto the opposed surface as a portion having a rod-like outer shape that is capable of being inserted into a bolt hole for mounting the cylinder head opening on the cylinder head mounting surface, and which has multiple bolt pieces that are formed on the lateral surface thereof with a threaded engageable to a female threaded portion of the bolt hole, characterized in that it forms a tapered bore portion that is tapered on the opposed surface side and that is opened on the opposite side of the opposed surface side by the internal surfaces of the bolt pieces, so as to expand the diameter thereof by displacing the multiple bolt pieces due to a wedge action receiving through the tapered bore portion; a tapered shaft member, which has a tapered surface portion that is inserted into the tapered bore portion with the tapered surface portion projecting from the tapered bore portion and that is configured to provide the wedge action by the relative movement between the bolt member thereof in the insertion direction; and a piston member, which is provided so that it can be biased by a given suppress strength in the direction projecting toward the opposed surface, and which has a pressing surface contacting a prescribed surface portion of the cylinder head mounting surface.