In-cylinder flow field in a spark-ignited natural gas engine was investigated by the laser light sheet PTV method, which can analyze flow field in a short time. Using a single-cylinder visualization engine, flow field was measured right below and 10 mm, 20 mm, and 40 mm below cylinder head, at various crank angles in intake, compression, and expansion strokes. The results showed that induction-generated swirl was getting concentric to the cylinder center in compression stroke, and was shifted to the outward radial direction in expansion stroke. Swirl velocity increases with the lapse of time, showing its maximum around 20° BTDC during compression stroke, and varies from cycle to cycle within ±5% to its maximum velocity. Squish velocity shows its inward maximum and outward maximum around 20° BTDC and 20° ATDC, respectively, and varies from cycle to cycle within ±8% to its maximum outward velocity. The maximum outward squish velocity in expansion stroke is about twice as large as the maximum inward squish velocity in compression stroke.