Provided is a discharge ionization current detector. To reduce the cost of a high-voltage power supply unit by reducing the discharge starting voltage for a low-frequency dielectric barrier discharge. Light from light source unit (20) located externally to cylindrical tube (2) through which a plasma gas (He) flows is irradiated through the wall surface of the cylindrical tube (2) onto a plasma generation region (the region between plasma generation electrodes (6 and 7) located in gas flow path (4). The light energy excites the He molecules or the minute quantities of impurity gas molecules present in the He gas, causing photo-ionization. This causes a discharge to start, and a plasma to be formed, when a low-frequency voltage of a voltage that is less than the usual discharge starting voltage is applied across electrode (5) and electrodes (6, 7). Once the discharge starts, the discharge is sustained when the usual discharge sustaining voltage is applied across electrode (5) and electrodes (6, 7). This means that the light source unit (20) has to be on only for a short duration when discharging is started.