A pulsing system of a slow positron beam has been developed to measure positron lifetime for thin films. The pulsing apparatus consists of positron storage section, chopper and bunching section. The storage section reduces the energy spread and makes a quasi-continuous beam. Positrons are bunched by suitable time-varying voltage which is produced by an arbitrary waveform generator and amplified. The time width of bunched positrons has reached similar to 550 ps (FWHM) with positron mean energy of similar to 100 eV at present.