A gas dissolution accelerating device with a simple structure efficiently increases the concentrations of oxygen dissolved in deep water, such as at lake bottoms. A gas dissolution accelerating device includes a cylindrical member located parallel to a vertical direction when installed, a box member having an opening facing downward when installed, and a fixing unit for fixing the box member to a diffuser. The box member includes a top plate having a cone-shaped protrusion protruding inward and a through-hole receiving the cylindrical member. The fixing unit includes a flat attachment plate having an upper surface onto which the box member is mounted, a pair of halved banding members for clamping a feeding pipe of the diffuser, and rod-shaped connecting members connecting the attachment plate to the banding members.