The present invention addresses the problem of providing an agent for improving sperm-motility, in particular, an agent for improving forward-sperm-motility, for use on reduced-motility sperm in male infertility treatment. Prepared is a gaseous agent for improving sperm-motility consisting of a gas that contains hydrogen molecules in the amount of 1% (v/v) or higher, for example, a gas that contains hydrogen molecules in an amount of between 45 and 55% (v/v), or a liquid agent for improving sperm-motility consisting of a liquid such as physiological saline, a culture solution, or a buffer solution containing hydrogen molecules in the amount of 1% or higher of the saturation solubility thereof, for example, a liquid in which hydrogen molecules have been dissolved by bubbling. The gaseous agent for improving sperm-motility can improve the motility of sperm by contacting hydrogen gas and either semen or a dilution thereof with each other in a gaseous phase. The liquid agent for improving sperm-motility can improve the motility of sperm when used as a washing solution or diluent for semen.
