Expecting that the laser propulsion is one of the most interesting next-generation space propulsion systems, it is important to analyze the mechanism of LSD (Laser-Supported Detonation) waves caused by laser absorption. The performance of laser propulsion is determined mainly by the laser absorption efficiency. We find that four physico-chemical processes take place in a LSD wave: (i) Laser energy is absorbed by free electrons through inverse bremsstrahlung. (ii) This energy is distributed to heavy particles (atoms and ions) through elastic and (iii) inelastic collisions, (iv) although it is partly lost as radiation emission by bremsstrahlung.We simulate this LSD wave, using 1-D and axi-symmetric 3-D models, and considering CO2 and CO lasers. Then we clarify the mechanism of LSD wave. Rapid radiation and ionization processes impose stiffness problems to the numerical analysis. To remove the stiffness, we have used a modified Harten-Yee-type TVD scheme which takes account of real gas effects.