Bring sugar and the water to a boil in large pan over medium heat; cook until sugar is completely dissolved. Reduce heat to low; add rhubarb. Cook until syrup is light pink and rhubarb is tender, about 5 minutes. Strain; refrigerate until ready to use, up to 5 days.Advertisement
Just before serving, combine syrup, shochu, and lime juice in a 2-quart pitcher, and refrigerate.
To serve, shake mixture in batches in a cocktail shaker filled with ice; pour into chilled rocks glasses.