The disclosed technology relates to medicinal fluid injection devices. In one aspect, a medicinal fluid injection device includes a housing having an opening in a side surface thereof, a storage container arranged inside the housing and configured to store a medicinal fluid, an injection needle connected to the storage container and configured to inject the medicinal fluid when stored in the storage container through the opening, and a cover body covering the opening, wherein the cover body includes a cover part configured such that the shape of the cover part changes according to air pressure caused by the injection needle.