This work is intended to study dynamic evolution of a magnetic flux tube that rises from an upper solar convection zone to a solar atmosphere, by performing a series of two-and-a-half dimensional MHD simulations focused on the circular cross section of the flux tube. In these simulaitons a cylindrical flux tube was initially placed horizontally in the convection zone (convectively unstable region), which started to rise and when the top of the flux tube reached a convectively stable solar surface (photosphere), the cross section changed the shape from an almost round one to a horizontally extended one (flattening of the flux tube), forming a magnetic layer just below the photosphere. A plasma inside the layer was squeezed out to the right and left sides of the layer, making it locally subject to the magnetic Rayleigh-Taylor instability. As the flattening of the flux tube proceeded, the undulation wavelength of the layer increased, and when it became longer than the critical wavelength for the instability (emergence wavelength), the flux tube started to emerge into the solar atmosphere. Subsequently, emergent part of the flux tube expanded upward when that part had sufficiently strong magnetic pressure compared to the gas pressure of a plasma lying on it. We confirmed that the expansion process was characterized by self-similar evolution (Shibata et al. 1990); that is, both the plasma and magnetic field had self-similar distributions in the emergent part.