A strain wave gearing comprising: a rigid internal gear; a flexible external gear disposed coaxially inside the internal gear; and a wave generator that flexes the external gear into a non-circular shape to partially mesh with the internal gear, and moves a meshing position between the two gears (3, 4) in a circumferential direction, wherein the external gear has a cylindrical external-tooth forming portion that is flexible in a radial direction and is provided with at least one end being an open end; and the wave generator is mounted coaxially inside the external-tooth forming portion, wherein the wave generator has: a wave-generator plug that is a rigid body having a non-circular contour; a wave-generator bearing mounted between a non-circular plug outer peripheral surface of the wave-generator plug and an inner peripheral surface of the external-tooth forming portion; and a foreign-matter intrusion preventing mechanism that covers a space formed between the wave-generator plug and an inner peripheral surface of the external gear, wherein the foreign-matter intrusion preventing mechanism has: an annular filter capable of capturing foreign matters mixed in a lubricant passing therethrough; and a lubricant-impermeable filter attachment plate, wherein the filter attachment plate is attached to the wave-generator plug so as to rotate integrally therewith; and the filter is attached to the filter attachment plate so as to rotate integrally therewith, and is in contact with the inner peripheral surface of the external-tooth forming portion in a slidable state, characterized in that the filter is provided with a non-circular filter outer peripheral surface similar in shape to a non-circular plug outer peripheral surface of the wave-generator plug, the filter is attached to the filter attachment plate so that the non-circular filter outer peripheral surface is in a same phase as the non-circular plug outer peripheral surface of the wave-generator plug, and the non-circular filter outer peripheral surface is maintained to be in contact with the inner peripheral surface of the external-tooth forming portion in a manner in which the filter is not subjected to radial flexing by the inner peripheral surface of the external-tooth forming portion. a rigid internal gear; a flexible external gear disposed coaxially inside the internal gear; and a wave generator that flexes the external gear into a non-circular shape to partially mesh with the internal gear, and moves a meshing position between the two gears (3, 4) in a circumferential direction, wherein the external gear has a cylindrical external-tooth forming portion that is flexible in a radial direction and is provided with at least one end being an open end; and the wave generator is mounted coaxially inside the external-tooth forming portion, wherein the wave generator has: a wave-generator plug that is a rigid body having a non-circular contour; a wave-generator bearing mounted between a non-circular plug outer peripheral surface of the wave-generator plug and an inner peripheral surface of the external-tooth forming portion; and a foreign-matter intrusion preventing mechanism that covers a space formed between the wave-generator plug and an inner peripheral surface of the external gear, a wave-generator plug that is a rigid body having a non-circular contour; a wave-generator bearing mounted between a non-circular plug outer peripheral surface of the wave-generator plug and an inner peripheral surface of the external-tooth forming portion; and a foreign-matter intrusion preventing mechanism that covers a space formed between the wave-generator plug and an inner peripheral surface of the external gear, wherein the foreign-matter intrusion preventing mechanism has: an annular filter capable of capturing foreign matters mixed in a lubricant passing therethrough; and a lubricant-impermeable filter attachment plate, an annular filter capable of capturing foreign matters mixed in a lubricant passing therethrough; and a lubricant-impermeable filter attachment plate, wherein the filter attachment plate is attached to the wave-generator plug so as to rotate integrally therewith; and the filter is attached to the filter attachment plate so as to rotate integrally therewith, and is in contact with the inner peripheral surface of the external-tooth forming portion in a slidable state, characterized in that the filter is provided with a non-circular filter outer peripheral surface similar in shape to a non-circular plug outer peripheral surface of the wave-generator plug, the filter is attached to the filter attachment plate so that the non-circular filter outer peripheral surface is in a same phase as the non-circular plug outer peripheral surface of the wave-generator plug, and the non-circular filter outer peripheral surface is maintained to be in contact with the inner peripheral surface of the external-tooth forming portion in a manner in which the filter is not subjected to radial flexing by the inner peripheral surface of the external-tooth forming portion.