In Part I of "Formal and Transcendental Logic" and related works Husserl had two different interests in his study of formal logic; one is the thechnical interest for which the problem on the use of "imaginary" concepts in mathematics is considered the "concluding theme", and the other is the logical interest for which the intentional analysis of formal logic from the natural attitude is considered the important preparation of his transcendental phenomenological study. The purpose of this paper is to clarify the role of the concept of definite multiplicity in the above two interests of his study. In particular, we show that the concept of definite multiplicity is the key concept to understand Husserl's view on his "logical" study as to (1) the relationship between logical deducibility and truth, (2) the relationship between logical deducibility and consistency, (3) the relationship between pure logic and mathematics, and (4) the relationship between formal syntax and "formal ontology". In the course of our investigation, we analyze the role of "formal ontology" from the two aspects, the technical aspect and the logical aspect. We also show how Husserl reached the solution of his "technical" problems and a concrete example of "definite multiplicity". We conclude this paper by discussing the possibility of the use of Husserl's method of intentional study in the contemporary "philosophy of mathematics".