Provided is a foamable filling device that includes a foamable material and is disposed within a hollow structural member. The foamable filling device includes a carrier member and an opening/closing member. The carrier member includes a through hole and a mounting portion to which the foamable material is mounted. The opening/closing member includes a cover portion formed in a size capable of blocking the through hole; and a biasing portion that biases the cover portion, and the opening/closing member is fittable to the carrier member. The opening/closing member is configured such that, when the opening/closing member is fitted to the carrier member, the biasing portion is elastically deformed and biases the cover portion to thereby bring the cover portion into contact with a seat portion provided at an end of the through hole of the carrier member.