A roots type fluid machine includes suction and discharge ports, rotary shafts and rotors. A pair of the rotors respectively has a number n of lobe and valley portions with apex and bottom ends for engaging each other. The lobe portions are located on imaginary lines extending radially from an axis of the rotary shaft. The outer surface of the rotor is defined by an outline of the rotor including an arc and involute and envelope curves being rotated and moved in the direction of the axis. The arc has a radius R and a center located on the imaginary line. The involute curve is formed by an imaginary base circle having a radius r and a center located at the axis. The envelope curve is formed by an arc having a radius R. The number n is four or more. A torsional ange ² is over 360 / n degrees.
