A novel manipulation of microfluid width in a microchannel was presented by controlling inflation of a gas boundary. The gas boundary was formed by heating water with a microheater in a semicircular shape from a chamber which was connected symmetrically to the microchannel. The formed gas boundary inflated perpendicularly to the flow direction and, consequently, the microfluid width was narrowed. The inflation and contraction were flexibly like a virtual wall and dependent on two factors: one is the flow velocity of the microfluid and the other is the pressure inside the gas boundary. Dimensions of the chamber and the microchannel width were determined empirically as same of $300;{mu}m$ for stable operation. The width of microfluid was manipulated manually with the microheater and could be maintained as up to $22;{mu}m$. The stable focusing began to be distorted when the flow velocity exceeded 17.8 mm/s.