This paper proposes a method for anaphora resolution of zero subjects in Japanese instruction manuals based on both the linguistic nature of expressions and the general ontology of the text type. In instruction manuals written in Japanese, zero subject is one of main reasons for ambiguity of sentences. In order to resolve them, the authors examined the property of several types of expressions including some forms of verbal phrases and some conjunctive clauses. As a result, they have a set of constraints and defaults for zero subject resolution. The authors verified the precision and recall rate of the constraints and defaults with real examples, and have found that they make good estimates with 97% precision and 80% recall.