To obtain an accurate spherical shaped grinding tool having a simple structure and a machining surface on which truing and dressing are finely performed.

In the cup type dresser 13 of a hollow cylindrical shape for truing and dressing a grinding tool 22 formed of abrasive grains 32 and a bond agent 33 uniting the abrasive grains 32 and having a machining surface 34 formed in a spherical shape, abrasive grains which have diameters smaller than the abrasive grains of the grinding tool 22, high frangibility, low hardness, and are harder than the bond agent 33 of the grinding tool 22 are united by holding force weaker than the abrasive grain holding force of the grinding tool 22. Since the holding force of the abrasive grains 37 constituting the cup type dresser 13 is weaker than the abrasive grain holding force of the grinding tool 22, the abrasive grains 37 of the cup type dresser 13 are dropped off due to a contact load with the grinding tool 22. The dropped-off abrasive grains 37 have smaller diameters than the abrasive grains 32 of the grinding tool 22 and harder than the bond agent 33 of the grinding tool 22, so that the dropped-off abrasive grains intrudes between the abrasive grains 32 on a grinding tool surface to selectively remove the bond agent 33 of the grinding tool 22 from the surface, thereby carrying out dressing.