A groove processing apparatus of processing a groove having a triangular cross section in a side of a plate material is provided. The groove processing apparatus includes a cutting member, a first driving part and a second driving part. The cutting member cuts the side of the plate material and the first driving part moves the cutting member to first and second positions. The second driving part moves the cutting member in groove processing directions of the plate material. The first position corresponds to any one of the two sides of the triangular shape having the same vertex and the second position corresponds to the other thereof, and the vertex is positioned inside the plate material.