A first communication passage is coupled to a left wheel cylinder pressure chamber at an upper vehicle side and a right wheel cylinder pressure chamber at a lower vehicle side to be communicated with each other, and a second communication passage is coupled to a left wheel cylinder pressure chamber at the lower vehicle side and a right wheel cylinder pressure chamber at the upper vehicle side to be communicated with each other. Between the first and second communication passages is a valve mechanism so that the first and second communication passages are normally communicated with each other, and fluid flow from one of the first communication passage and the second communication passage to the other is blocked, when a pressure difference, caused when fluid moves from one of the first communication passage and the second communication passage to the other, becomes equal to or greater than a predetermined value.