To efficiently and safely make an opening on an inner wall or the like of a building with less time and labor as compared with conventional one.

The portable electric multi-blade circular saw comprises two sheets or more of circular saws are fixed to one rotation shaft with a predetermined gap, a nut part is fixed to one end of the rotation shaft and a mounting socket is fixed to the other end; an electric part connected to a power socket fitted to the nut part and transmitting rotation force to the rotation shaft; and a grip part fitted to the mounting socket during operation and pivotally supporting the rotation shaft. The circular saw has the same shape and/or one circular saw is slightly larger than the other. The number of multi-blades is made to 2 or more, i.e., a plurality of numbers depending on an object to be worked. A cutting straight line is not deviated and it is a specification for an easy work. A mutual gap of the circular saws is variable in order to adjust a shape, a kind and a purpose of the opening to the optimization.