Wash and strain the adzuki beans. In a large pot, add adzuki beans and pour water until 1 inch above the beans. Bring it to a boil over medium high heat for 5 minutes. Drain the beans and discard the water.

Return the beans to the pot and pour water until 1 inch above the beans. Cover, and simmer over medium low heat until the beans are soft and easily breakable for about 1 to 1 1/2 hours. Turn off the heat, and let it cool for about 20 minutes.

Strain and discard the liquid. Using a hand blender, mash the bean coarsely. You may add little bit water to help mashing the beans. 

Put the mashed beans back into the pot in medium heat. Add granulated sugar in 3 batches while stirring constantly. Taste to adjust the sweetness each time you add each batch. Add salt and turn off the heat.

Let it cool completely before storing red bean paste into the fridge.