Welding of different fluoroplastics was performed by infrared laser. Aiming to achieve overlap welding without deformation using heat transfer technology, we conducted surface cooling and inside heating during laser irradiation using a laser transparent heat spreading body. This paper reports on the characteristics and optimum conditions of welding in an experiment targeted at welding the different fluoroplastics of PFA and PTFE. Welding of PFA and PTFE sheets using a CO_２ laser was performed and conditions were analyzed by numerical calculation of heat transfer within a model of two piled sheets and cross-sectional observation of the welded test pieces (optical microscope observation). Furthermore, ingredient analysis by laser Raman microscope was carried out. Then, strength evaluation by tensile test was attempted. Overlap welding of PFA and PTFE sheets without surface damage was successfully conducted. The result of measuring tension strength for the PFA and PTFE sheets indicated equal strength to that when welding PFA sheets. In particular, the deviation in applied energy density arising from laser beam irradiation of Gaussian distribution strength caused the temperature increase inside the resins to deviate, indicating a spectrum in which the characteristics of both materials were averaged over a certain range in the vertical direction across the interface position at the center of the laser spot. It was thus inferred that sufficient mutual penetration had occurred. On the other hand, unique spectra of both materials were observed at the circumference part of the beam spot with low energy density, suggesting that the energy required for welding was insufficient. These results showed correlation with the results of investigating the effect of the intensity distribution in the beam spot of the temperature distribution by heat transfer analysis.