The 248 nm ablation of poly(methyl methacrylate) (PMMA) doped with 5-diazo Meldrum’s acid (DM) has been investigated by measurements of etch profile, transient absorption spectra, and transient absorbance at 248 nm. There were three fluence regions with respect to characteristic morphological changes of the polymer film; no morphological change, an elevation of the film surface (swelling), and etching were observed in the low, medium, and high fluence regions, respectively. The photodecomposition of DM into the ketene was confirmed by transient absorption spectroscopy, and its yield was determined to be 0.62. The results obtained by transient absorption spectroscopy made it possible to simulate the photodecomposition dynamics of DM, and the amounts of the absorbed energy and the photodecomposition products were estimated. On the basis of the obtained results, it is concluded that the amount of the absorbed energy governs ablation; namely, thermal processes are dominant although DM photodecomposes with a high quantum yield. The cyclic multiphotonic absorption by the ketene is discussed.