The viscosity of molten metal is an important factor for calculating flow lines. The aim of this study was to obtain the viscosity of non-combustible magnesium alloy using the theoretically established Hagen-Poiseuilles law. Three kinds of metals; pure aluminum (Al), Al-Si alloy (AC4C), and non-combustible magnesium alloy (Mg-9mass%Al-1mass%Zn-2mass%Ca (were used to measure viscosity. The pressure difference between both ends of the tube was calculated by the hydrostatic stability of gravity (HSG method) as well as by the measured pressure (MP method). The viscosity was obtained from Hagen-Poiseuilles equation. The velocity of the metals when pumped into the silica tube was constant. Therefore, flow was steady in the tube. The viscosity calculated using the MP method was relatively high compared with the HSG method, and consequently showed higher accuracy. This indicates that the MP method provides more reliable viscosity values for non-combustible magnesium alloys.