Rinse the brown rice thoroughly and soak for at least 30 minutes.
Slice the shiitake mushrooms thinly, and cut the kelp and hijiki into bite-sized pieces.
Place the soaked brown rice, shiitake mushrooms, kelp, and hijiki into a pot.
Pour water over the ingredients and cover with a lid.
Boil over high heat; once it starts boiling, reduce to low heat and cook for 30 minutes.
Turn off the heat, let it steam for 10 minutes, gently mix, and serve in a bowl.