To make assembling work efficient by allowing an E-ring to be assembled with one hand.

In this E-ring assembling device, the tip of an oscillating arm 6 is integrally provided with an E-ring setting member 10 for suction-holding an E-ring 45 and pressing the E-ring 45 downward, an E-ring delivery mechanism part 20 for delivering the E-rings 45 one by one into the suction-holding position of the E-ring setting member 10, and an E-ring opening guide member 40 positioned below the E-ring setting member 10 and guiding the E-ring 45 lowered being pressed by the E-ring setting member 10, so that its diameter is enlarged.