The present invention relates to a method for detecting target nucleic acids by simultaneous isothermal amplification of the target nucleic acids and a signal probe 5 using an external primer set, a DNA-RNA-DNA hybrid primer set and a DNA-RNA-DNA hybrid signal probe. The method according to the present invention can be used to amplify target nucleic acids in a sample, rapid and exact manner without the risk of contamination compared to the conventional methods such as PCR, and it can simultaneously amplify target nucleic acid and a signal probe, so that it can 0 be applied to various genome projects, detection and identification of a pathogen, detection of gene modification producing a predetermined phenotype, detection of hereditary diseases or determination of sensibility to diseases, and estimation of gene expression. Thus, it is useful for molecular biological studies and disease diagnosis.