House of String Brace is a co-working space on the ground floor and the shared flat on the first floor in the two storey house which was used to be two dormitories with separate entrances. The site is on narrow alleys in Takanawa area in Tokyo, where many temples remain. The original house has timber structure of 60 years old and is surrounded by houses, apartments and cemetery. The north side where general light from the cemetery is obtained becomes co-working’s free space, and it connects the front alley to the back alley. In order to make the free space as transparent as possible, it is reinforced seismically with “string brace”. Since the string made of high-strength aramid fiber has a tensile strength of half the steel bar of 9mm, it gives strength equal to that of the steel brace by multiplying the string twice. The string is soft to be constructed easily by tying and can be wound small to transport. Also, the softness of the string gives a gentle impression to the living space. To seek a way to construct by ourselves without resorting to industrial products, it would be lead to widen more the possibility of seismic reinforcement and architectural design.