This paper presents MIDAS (Media Interaction Design Authoring System), an authoring toolkit for designers and artists to develop working prototypes in new interaction design projects. Field research were conducted to identify the requirements and a case study of designing new interactive products was carried out to examine the feasibility of the new tool. MIDAS provides easier ways of integrating hardware and software, to manage a wide range of electric input and output elements and to employ 3D Augmented Reality technology within conventional multimedia authoring tools, such as Director and Flash, which are popularly used by designers. MIDAS was used in case study projects of design education as well as by voluntary designers for evaluation. From the result of case studies, it was found that many design projects were successfully accomplished using MIDAS. Designers who participated in the projects reported that MIDAS not only helped them to concentrate more on ideation but also was very easy to use as they implemented the physical interface concepts without advanced engineering skills. It is expected that MIDAS can also support prototyping in interactive media an, tangible user interface development and related human computer interaction fields.