A replaceable orifice unit is disposed outside a pressure or a flow regulator valve, and therefore easy in maintenance, requires the smallest possible number of components, is compact in construction to save space, free from any fear that fluid impurities in a fluid circuit bank up against an upstream side of an orifice piece (2) of the unit, advantageous in replacing one type of chemicals flowing through the circuit with another type, and also advantageous in manufacturing cost. The replaceable orifice unit is constructed of a tube member (1), in which the orifice piece (2) assuming two symmetrically arranged similar triangular shapes in longitudinal cross section is fixedly mounted. Formed between vertexes of the triangle shapes is an orifice (2a). An O-ring is coaxially disposed between the tube member (1) and the orifice piece (2) to prevent the fluid from passing through therebetween.