Various software testing methods for an embedded software have been proposed so far but there has been a lack of 100% code coverage for them. Existing commercial testing tools may not be effective in applying to every safety-critical system’s software which needs to be certifiable up to DO-178B level A (for instance, aviation or defense). The objective of this paper is to develop a new unit test method for the safety-critical software. This paper presents a new test procedure using simulation of an IDE Tool provided by a chipmaker and an automated test case generator named Make-Test tool which works with Excel Macro. It is designed to be able to verify untested source codes as well as branch coverages and can be utilized for the safety-critical software.