Use of easily handled, liquid or solid carbon monoxide (CO) sources instead of toxic CO gas can contribute to reduce the risk and anxiety of accidents in organic synthesis, thus accelerating the research in carbonylative transformations and increasing the diversity of chemical products. Based on the discovery that phenyl formate could generate CO in the presence of a weak base, we considered utilization of phenyl formate as a CO source to develop “external-CO-free” synthetic methods of high safety and practicality. This concept led us to develop a series of Pd-catalyzed carbonylative transformations with formic acid derivatives as CO sources. In parallel, 2,4,6-trichlorophenyl formate and N-formylsaccharin were developed as highly crystalline but highly reactive CO sources. These novel reactions without use of CO gas efficiently afforded various carbonyl compounds in a one-pot manner in good to excellent yields. We also demonstrated gram-scale synthesis using ordinary glassware, which showed high practicality and feasibility for further scale-up.