Although the formation of nanographenes such as graphene nanoribbons (GNRs) has been widely investigated, the formation methods are still severely limited by the lack of a suitable method for applying them. To address this, we investigated the use of soft X-rays to form GNRs. Precursor films were prepared by hot mesh deposition (HMD) using pentacene molecules and H2 gas. Various nanocarbon films, namely pentacene, 6,13-dihydropentacene, and a mixture of pentacene, 6,13-dihydropentacene, and pentacene oligomers were prepared by HMD then irradiated using soft X-rays generated in the NewSUBARU synchrotron radiation facility. The properties of the nanocarbon films synthesized from pentacene, 6,13-dihydropentacene, and pentacene oligomers by soft X-ray irradiation differed. The hydrogen desorption and catalytic effect of the Ni film on the quartz substrate during soft X-ray irradiation enhanced the polymerization of pentacene. The pentacene oligomers acted as nuclei for graphene formation. Although GNRs were not synthesized by soft X-ray irradiation, graphene was obtained with a high yield at a low temperature of 760 °C. Nanographene synthesis from pentacene by soft X-ray irradiation was investigated to control the molecular structure and improve the yield of nanographene. Nanographene was not obtained from pentacene and 6,13-dihydropentacene by soft X-ray irradiation. By using a nanocarbon film with pentacene oligomers prepared on a Ni film using a WNi(3%) mesh prepared by HMD, graphene was produced by soft X-ray irradiation instead of nanographene.