The photon-number distribution of an output state from a single-photon source using gated spontaneous parametric downconversion is analyzed in detail. For an improved experimental result of {P(0),P(1),P(2)} ={0.59,0.40,0.009}, it is found that P(1) is restricted by the optical loss of the signal photon path (62%) and the cause of nonzero P(2) is the imperfection of the optical shutter for gating. A method by which to reconstruct the photon-number distribution via photon-counting measurement is also presented. (c) 2005 Optical Society of America.