A drug injection device, comprising: an injection unit which is provided with an ampoule accommodation portion for accommodating an ampoule that contains an injectable solution, and injects the injectable solution into a person being operated on; and an injection speed adjustment unit which is connected to the injection unit, applies pressure to the injectable solution, and adjusts the injection speed of the injectable solution, being injected into the body of the person being operated on, by selectively adjusting the speed of the pressure applied to the injectable solution, wherein the injection speed adjustment unit comprises: an injection unit attachment portion to which the ampoule accommodation portion is detachably attached; a pressurizing plunger which is connected to the ampoule accommodated in the ampoule accommodation portion and applies pressure to the injectable solution; and a pressurizing plunger up/down driving module which is provided on the same axis as the pressurizing plunger and drives the pressurizing plunger up or down; characterized in that: the injection speed adjustment unit further comprises a waterproof portion through which the pressurizing plunger passes and that prevents a liquid from penetrating to the inside of the injection speed adjustment unit, and the waterproof portion comprises: a waterproof portion body with a through-hole through which the pressurizing plunger passes; and a watertight member supported by the waterproof portion body and sealing a gap between the outer wall of the pressurizing plunger and the side wall of the through-hole. an injection unit attachment portion to which the ampoule accommodation portion is detachably attached; a pressurizing plunger which is connected to the ampoule accommodated in the ampoule accommodation portion and applies pressure to the injectable solution; and a pressurizing plunger up/down driving module which is provided on the same axis as the pressurizing plunger and drives the pressurizing plunger up or down; characterized in that: the injection speed adjustment unit further comprises a waterproof portion through which the pressurizing plunger passes and that prevents a liquid from penetrating to the inside of the injection speed adjustment unit, and the waterproof portion comprises: a waterproof portion body with a through-hole through which the pressurizing plunger passes; and a watertight member supported by the waterproof portion body and sealing a gap between the outer wall of the pressurizing plunger and the side wall of the through-hole. the injection speed adjustment unit further comprises a waterproof portion through which the pressurizing plunger passes and that prevents a liquid from penetrating to the inside of the injection speed adjustment unit, and the waterproof portion comprises: a waterproof portion body with a through-hole through which the pressurizing plunger passes; and a watertight member supported by the waterproof portion body and sealing a gap between the outer wall of the pressurizing plunger and the side wall of the through-hole. a waterproof portion body with a through-hole through which the pressurizing plunger passes; and a watertight member supported by the waterproof portion body and sealing a gap between the outer wall of the pressurizing plunger and the side wall of the through-hole.