A back brace includes a first brace member having a first connector, and a second brace member having a second connector and a coupler at two opposite sides thereof. The coupler is detachably connectable to the first connector. A first adjustment holder is affixed to one end of the first brace member and holds therein a first rod member. A second adjustment holder is affixed to one end of the second brace member and holds therein a second rod member. Two pull cords are inserted through the first adjustment holder and the second adjustment holder and run alternately back and forth over the first rod member and the second rod member to adjustably and abuttably hold the first and second brace members side by side. Two fastening members are fastened to distal ends of the pull cords and detachably connectable to the first and second connectors.