A method for detecting a target gene using a microfluidic device for detecting the target gene, the method comprising: providing the microfluidic device for detecting the target gene to comprise -- a board; -- an inlet which is formed on the board and through which a sample solution is introduced from the outside; -- a first channel connected to the sample inlet to accommodate the introduced sample solution; -- a second channel connected to the first channel; -- an outlet connected to the second channel; -- a coating on the second channel; a primer immobilized on the coating of the second channel; and -- a template complementarily binding to the primer, wherein the template comprises binding regions complementary to a target gene, a binding region complementary to the primer, and in-template complementary binding regions suitable to form a dumbbell shape, the binding regions complementary to the target gene are separately formed at both ends of the template, and the binding region complementary to the primer is formed between the in-template complementary binding regions suitable to form the dumbbell shape, which are separately formed, introducing a sample solution into the first channel, adding a dNTP, a ligase, and an isothermal nucleic acid ploymerase to the second channel for rolling circle amplification, and detecting the target gene by allowing amplified gene products formed by the rolling circle amplification to flocculate by forming hydrogel on the second channel and the outlet and thus blocking the second channel and the outlet. providing the microfluidic device for detecting the target gene to comprise -- a board; -- an inlet which is formed on the board and through which a sample solution is introduced from the outside; -- a first channel connected to the sample inlet to accommodate the introduced sample solution; -- a second channel connected to the first channel; -- an outlet connected to the second channel; -- a coating on the second channel; a primer immobilized on the coating of the second channel; and -- a template complementarily binding to the primer, wherein the template comprises binding regions complementary to a target gene, a binding region complementary to the primer, and in-template complementary binding regions suitable to form a dumbbell shape, the binding regions complementary to the target gene are separately formed at both ends of the template, and the binding region complementary to the primer is formed between the in-template complementary binding regions suitable to form the dumbbell shape, which are separately formed, -- a board; -- an inlet which is formed on the board and through which a sample solution is introduced from the outside; -- a first channel connected to the sample inlet to accommodate the introduced sample solution; -- a second channel connected to the first channel; -- an outlet connected to the second channel; -- a coating on the second channel; a primer immobilized on the coating of the second channel; and -- a template complementarily binding to the primer, wherein the template comprises binding regions complementary to a target gene, a binding region complementary to the primer, and in-template complementary binding regions suitable to form a dumbbell shape, the binding regions complementary to the target gene are separately formed at both ends of the template, and the binding region complementary to the primer is formed between the in-template complementary binding regions suitable to form the dumbbell shape, which are separately formed, introducing a sample solution into the first channel, adding a dNTP, a ligase, and an isothermal nucleic acid ploymerase to the second channel for rolling circle amplification, and detecting the target gene by allowing amplified gene products formed by the rolling circle amplification to flocculate by forming hydrogel on the second channel and the outlet and thus blocking the second channel and the outlet.