The present invention provides a method for measuring a target gene under isothermal conditions without using enzyme. A pair of probes each having n (n.gtoreq.3) base sequence regions complementary to each other are hybridized alternately to form a double-stranded probe-polymer. A base pair at branched sites of each complementary base sequence region is designed to be a G (guanine)-C (cytosine) bond, whereby a stable double-stranded probe-polymer is formed. One of complementary portions in one probe is constituted to have a base sequence complementary to a part of a target gene, whereby a target gene-probe-polymer complex is formed and the target gene is measured.