In [Kachi 2002a] I made comparison between several definitions of validity in Simple Partial Logic (SPL) and adopted two of them as most appropriate. In this paper, after elaborating more on these two definitions than in my previous paper and considering the characteristics of Partial Semantics, in which these definitions are given, I construct a tableau proof theory and prove its soundness and completeness. Then, based on the characterization of Partial Semantics, I will show that we can regard SPL as a logic of extensional alethic modality.