It has been reported that Japanese distributive numeral "zutsu" induces two types of distributive readings, namely an individual distributive reading and an occasion distributive reading. However, there is another reading, which evaluates distributivity in a different situation from a situation in which the rest of the clause is evaluated. I call it a group distributive reading. In this paper, I aim for a unified account of these readings. I propose that "Num-CL-zutsu" partitions a situation along with a presupposed function so that each sub-situation contains a unique/maximal individual which satisfies the measure phrase "Num-CL." The distribution of the three readings follows from an independently motivated constraint on situation pronoun binding. The semantic difference among the three readings comes from different ways to identify the presupposed function from situations to individuals.