Thermal convection in liquid gallium layer was investigated as a fundamental study for Rayleigh-Benard convection in low Prandtl number fluid using ultrasonic velocity profiler, UVP. Temperature dependence of sound speed in liquid gallium was measured for a velocity profile measurement by UVP. Measured velocity profiles visualized a convective motion in the liquid gallium layer, and it is a roll shape motion. Spatio-temporal velocity profile showed that there are two types of spatial fluctuation of the convection roll, and there are 3 modes on the number of convection rolls in the fluid layer, 2, 3 or 4 rolls. Two axes-simultaneous velocity profile measurement clarified that expand and contraction of convection roll has a phase delay on the direction of the rotating axis of the roll.