To provide a reinforcement for joining which can join the corner parts of a picture frame, furniture and building materials accurately and easily at a prescribed angle (corner part angle) and reinforce them firmly.

This reinforcement, aiming at incorporation of a bent reinforcement in the corner part, has a first arm part 11, which is inserted in a first insertion hole 18 at a right angle to a first outer side surface 8 and also has a first screw part 16, a second arm part 12 which is inserted in a second insertion hole 19 at a right angles to a second outer side surface 9 and also has a second screw part 17, and a connection part 13 which connects the first and the second arm part 11, 12 at a prescribed bending angle. The reinforcement is constituted of a first bolt 14 which is engageable with the first screw part 16 of the first arm part 11 and of a second bolt 15 engageable with the second screw part 17 of the second arm part 12. In this way, a first and a second reinforced material 2, 3 can be joined to each other.