A quasi‐continuous KrF(248 nm:B→X)fluorescence of over 8 ms pulse duration with a 100 Hz operation frequency was observed in a microwave‐discharge‐pumped KrF lamp with an Ar buffer gas in place of the He/Ne buffer. With an average microwave power deposition of 517.1 W, the maximum average KrF fluorescence power was 58.2 W obtained at a KrF intrinsic efficiency of 11.2% with the lamp gas mixture of F2/Kr/Ar=4/2/94(%) at 50 Torr and simultaneously an average ArF(193 nm:B→X) fluorescence power of 17.4 W with over 8 ms pulse duration at a 100 Hz operation frequency was also obtained. Based on the total fluorescence power of ArF and KrF, the intrinsic efficiency becomes as high as 14.6%. The maximum KrF intrinsic efficiency was 13.8% obtained with a lamp gas mixture of F2/Kr/Ar=2/2/96(%) at 50 Torr. The maximum average KrF fluorescence power and KrF intrinsic efficiency with the Ar buffer has exceeded the maximum average KrF fluorescence of 53 W and the corresponding KrF intrinsic efficiency of 8.3% achieved previously with the lamp gas mixture of F2/Kr/He/Ne=2/1/48.5/48.5(%) at 50 Torr. Dual wavelength emission should be useful for those that need both the 248 and 193 nm wavelengths.