PRE-CLEANER

PROBLEM TO BE SOLVED: To provide a pre-cleaner capable of removing foreign substances from suction air with high efficiency regardless of a rotating speed of an engine.SOLUTION: A pre-cleaner 1 is provided at an upstream side of an air cleaner in an intake duct where suction air flowing into an engine is circulated. The pre-cleaner 1 comprises: a double cylinder part 20 which is provided at the downstream side; and a spiral flow forming section 10 which is provided at an upstream side for forming a spiral flow by applying a stirling component to suction air. The double cylinder part 20 includes an inner cylinder 21, an outer cylinder 22 and a discharge port D communicating a space S between the inner cylinder 21 and the outer cylinder 22 and the outside of the intake duct. The spiral flow forming section 10 includes a fin 12 having an inclination angle in the direction of a central axial line C of the intake duct, and a fin operation mechanism 13 for changing spiral flow characteristics of suction air by changing a position or an attitude of the fin 12 in accordance with the engine-side negative pressure.