Place rice into a large, deep bowl. Fill with cold water and rub rice together with hands until the water turns milky white. Pour off the cloudy water, being careful not to pour out the rice. Repeat 3 or 4 times until you can see the rice through 8cm of water.
Drain the rice in a fine strainer, then place into a saucepan along with konbu and water. Allow to stand for 30 minutes. Stir together rice vinegar, sugar, and salt until dissolved in a small bowl, set aside.
Cover, and bring rice to a boil over high heat, then reduce heat to low, and simmer for 15 minutes. Remove from heat and allow to stand for 5 minutes.
Scrape rice into a bowl; remove and discard the konbu. Stir in vinegar mixture until well incorporated and no lumps of rice remain. Allow to cool at room temperature. For a shinier appearance, use an electric fan to cool the rice rapidly.