To enable the presence or absence of a specific base sequence to be accurately judged in a short time.

A DNA sample is subjected to a PCR method by using a dNTP bonded to a magnetic fine particle, and a fluorescent labelled primer. The product obtained by the PCR method is moved by electrophoresis in a gel 8 to a positive pole 4 while generating magnetic field by a magnet 6 present at a position apart from the positive pole 3, and the distribution of fluorescence intensity on the gel 8 is observed.