Embodiments of this application provide a balance-unbalance conversion apparatus. The apparatus includes an insulation substrate, a first microstrip, a second microstrip, and a conductive ground. The first microstrip includes a first balance signal connection section, a first impedance matching section, and an unbalance signal connecting section. The unbalance signal connecting section is configured to transmit an unbalance signal. The second microstrip includes a second balance signal connecting section, a second impedance matching section, and a ground section. The second balance signal connecting section is configured to transmit a second component of the balance signal. The ground section is configured to connect to a ground signal. The first microstrip, the second microstrip, and the conductive ground are all disposed on the insulation substrate, and a cross-sectional area of at least a part of the first microstrip and/or at least a part of the second microstrip is gradient.