For ginger juice, grate the fresh ginger and then squeeze the pulp to make the juice.
Put the ground chicken in the frying pan and then add the sake and sugar. Stir and break up the clumps and then cook over medium heat, stirring and breaking up the meat, for a few minutes, until the meat turns white.
Add the soy sauce and simmer a few more minutes, stirring, another 2-3 minutes, then add the ginger juice and cook, stirring, another minute or so.