1. Peel the bonito skin, cut into half inch cubes. Sprinkle with salt. Leave for approximately 10 minutes. Drain any liquid.
2. In a medium sized pan, bring water to a boil. Place bonito in water and boil until color changes, then drain in a bamboo colander.
3. Peel the ginger and cut in julienne strips.
4. Heat soy sauce, mirin, sake and sugar in a pan until boiling. Add bonito and ginger. Turn to medium heat and simmer until soup is gone.