Leading design is a concept to support designing a highly advanced artifact for which necessary technical seeds are not sufficiently developed. Leading design allows a designer to complete a design process by using virtual technical seeds to avoid the deadlock caused by the shortage of required technical seeds. As the result, the designer should obtain an overall view on what are actually required to what extent to achieve the goal. Introducing virtual technical seeds, however, causes problems such as explosion and uncertainty of design solutions in an extremely broad design space. This paper proposes a method of solving those problems and embodying the “leading design” concept as a practicable design method. By defining a development “hardness” of virtual technical seeds and using it as an objective function for an optimization, leading design solution can be converged to a virtual but least-difficult one. Also, probability distribution is introduced to give a leading design solution probability estimation. The embodied method is applied to a leading design of a micro factory component.