Abstract: The surface of a central section of a spiral element formed on a stationary or a movable scroll unit of a scroll type compressor is determined through the steps of locating an outer wall surface changing point G in a first polar coordinate system X, Y having a first origin O, drawing a circle Cs with its center at the first origin O and with a diameter corresponding to the orbital radius e of the orbiting motion of the movable scroll unit, locating a contact point P at a desired contact angle .beta. on the circle Cs, drawing a straight line m passing through the contact point P and the first origin O, drawing a straight line I passing the outer wall surface changing point G and inclined at a predetermined angle .alpha.