A silk hat-type strain wave gear device comprising: a rigid internal gear; an external gear having a silk hat shape, the external gear being provided with a flexible cylindrical body coaxially arranged inside the internal gear, external teeth formed on the cylindrical body, a diaphragm extending outward in a radial direction from an end of the cylindrical body, and an annular rigid boss formed continuously on an outer peripheral edge of the diaphragm; a wave generator for flexing the cylindrical body of the external gear into a non-circular shape to mesh the external teeth with internal teeth of the internal gear and for moving a meshing position of the external teeth with the internal teeth in a circumferential direction; a bearing for supporting the internal gear and the external gear in a relatively rotatable state; a first gap part and a second gap part, the first gap part being formed between inner and outer races (51, 52) of the bearing and being opened in one first end face of the bearing, the second gap part being formed between the inner and outer races (51, 52) of the bearing and being opened in the other second end face of the bearing; and an oil seal for sealing the first gap part; characterized in that the silk hat-type strain wave gear device further comprises: a gear-side gap part having an annular cross-section, the gear-side gap part extending along the diaphragm and the cylindrical body from the outer peripheral edge of the diaphragm of the external gear to a meshing section between the external teeth and the internal teeth; and a bearing seal for separating the second gap part from the gear-side gap part, wherein the bearing seal seals an opening end of the second gap part communicating with the gear-side gap part from a side of the gear-side gap part, for preventing lubricant from flowing into the second gap part from the gear-side gap part and for allowing lubricant to flow out to the gear-side gap part from the second gap part. a rigid internal gear; an external gear having a silk hat shape, the external gear being provided with a flexible cylindrical body coaxially arranged inside the internal gear, external teeth formed on the cylindrical body, a diaphragm extending outward in a radial direction from an end of the cylindrical body, and an annular rigid boss formed continuously on an outer peripheral edge of the diaphragm; a wave generator for flexing the cylindrical body of the external gear into a non-circular shape to mesh the external teeth with internal teeth of the internal gear and for moving a meshing position of the external teeth with the internal teeth in a circumferential direction; a bearing for supporting the internal gear and the external gear in a relatively rotatable state; a first gap part and a second gap part, the first gap part being formed between inner and outer races (51, 52) of the bearing and being opened in one first end face of the bearing, the second gap part being formed between the inner and outer races (51, 52) of the bearing and being opened in the other second end face of the bearing; and an oil seal for sealing the first gap part; characterized in that the silk hat-type strain wave gear device further comprises: a gear-side gap part having an annular cross-section, the gear-side gap part extending along the diaphragm and the cylindrical body from the outer peripheral edge of the diaphragm of the external gear to a meshing section between the external teeth and the internal teeth; and a bearing seal for separating the second gap part from the gear-side gap part, a gear-side gap part having an annular cross-section, the gear-side gap part extending along the diaphragm and the cylindrical body from the outer peripheral edge of the diaphragm of the external gear to a meshing section between the external teeth and the internal teeth; and a bearing seal for separating the second gap part from the gear-side gap part, wherein the bearing seal seals an opening end of the second gap part communicating with the gear-side gap part from a side of the gear-side gap part, for preventing lubricant from flowing into the second gap part from the gear-side gap part and for allowing lubricant to flow out to the gear-side gap part from the second gap part.