Nitrogen laser induced two photon ionization spectrometric (TPIS) signals were detected with a simple dc current detector (picoammeter), of which the detection power is nearly the same as with a boxcar peak-gated detection technique. The advantages of this dc detection system are the simple operation and low cost of construction, i.e., no special instruments are necessary to obtain the TPIS signal. By monitoring TPIS and background (leakage) currents alternately, the present dc system is successfully applied to the detection of the oil-soluble vitamins at ppb to sub ppb levels.