Non-contact and non-destructive usages of laser ultrasonic have been researched actively nowadays. Specifically, there are some studies about Lamb wave inverse problems, which is the research about inversely calculating material properties from Lamb wave dispersion curve. In this study, measurement system of Lamb wave phase velocity and method of inversely calculating elastic modulus of isotropic metal plate from the experimental dispersion curve of A0 mode. Ultrasounds are induced with a pulsed laser and detect them with a laser Doppler vibrometer, and measured the velocities of them in non-contact and non-destructive way. Then, calculating method of phase velocity distribution of A0 mode is introduced. By using characteristic equation of A0 mode, the sum of squares of residuals, SSR, is defined to represent how close the bulk wave velocities expectations are close to the right values when dispersion curve is experimentally given. Also, this study verified that assuming Poisson's ratio cannot affect inverse calculation results, which allows to roughly assume the ratio for the calculation. For the aluminum 0.5 mm plate, Lamb wave measurement system, phase velocity calculation method, and inversely calculation method by assuming Poisson's ratio roughly are verified by success of inversely calculating the elastic modulus of aluminum specimen accurately.