A method of detecting cytokeratin 19 mRNA in a sample employing an RNA amplification process, said method consisting of the following steps: synthesizing a cDNA with an enzyme having RNA-dependent DNA polymerase activity, by using CK19 mRNA as a template, and a primer consisting of the oligonucleotide listed as SEQ ID NO: 33, thereby producing a double-stranded RNA-DNA; decomposing the RNA of said double-stranded RNA-DNA obtained in above by use of an enzyme having ribonuclease H (RNase H) activity, thereby forming a single-stranded DNA complementary to said specific base sequence; forming double-stranded DNA with an enzyme having DNA-dependent DNA polymerase activity, using a primer consisting of the oligonucleotide listed as SEQ ID NO: 23 and that includes an RNA polymerase promoter sequence at the 5'-end thereof, and said single-stranded DNA obtained in as a template; forming an RNA transcription product derived from said specific base sequence with an enzyme having RNA polymerase activity, using said double-stranded DNA having said promoter sequence obtained in above as template; allowing said RNA transcription product derived from said specific base sequence obtained in to serve as a template for the synthesis of a new double-stranded DNA by the first and second primers, and allowing said reactions to to proceed to cause an RNA amplification in a chain reaction to obtain an amplified RNA transcription product derived from said specific base sequence and, detecting said amplified RNA transcription product by detecting a change in fluorescent properties in the presence of a fluorescent dye-labeled oligonucleotide probe designed to change its fluorescent properties when it forms a complementary double-strand with the target RNA. synthesizing a cDNA with an enzyme having RNA-dependent DNA polymerase activity, by using CK19 mRNA as a template, and a primer consisting of the oligonucleotide listed as SEQ ID NO: 33, thereby producing a double-stranded RNA-DNA; decomposing the RNA of said double-stranded RNA-DNA obtained in above by use of an enzyme having ribonuclease H (RNase H) activity, thereby forming a single-stranded DNA complementary to said specific base sequence; forming double-stranded DNA with an enzyme having DNA-dependent DNA polymerase activity, using a primer consisting of the oligonucleotide listed as SEQ ID NO: 23 and that includes an RNA polymerase promoter sequence at the 5'-end thereof, and said single-stranded DNA obtained in as a template; forming an RNA transcription product derived from said specific base sequence with an enzyme having RNA polymerase activity, using said double-stranded DNA having said promoter sequence obtained in above as template; allowing said RNA transcription product derived from said specific base sequence obtained in to serve as a template for the synthesis of a new double-stranded DNA by the first and second primers, and allowing said reactions to to proceed to cause an RNA amplification in a chain reaction to obtain an amplified RNA transcription product derived from said specific base sequence and, detecting said amplified RNA transcription product by detecting a change in fluorescent properties in the presence of a fluorescent dye-labeled oligonucleotide probe designed to change its fluorescent properties when it forms a complementary double-strand with the target RNA.