A prefilled and before-use mixing syringe comprising: an outer cylinder; a hollow needle provided in the outer cylinder as extending axially and having a cutting edge at its front end; a first gasket provided in the outer cylinder on a front side of the hollow needle; and a second gasket provided in the outer cylinder on a front side of the first gasket; wherein a first space containing a liquid is defined in the outer cylinder on a rear side of the first gasket, and communicates with a rear end of the hollow needle, the syringe further comprising: non-rotatively and complementarily shaped first and second engagement portions (12, 13) respectively provided at a front end of the first gasket and a rear end of the second gasket; and characterized in that it further comprises a second space containing a drug to be mixed with the liquid is defined between the first gasket and the second gasket in the outer cylinder with a front end face of the first engagement portion and a rear end face of the second engagement portion abutting against each other to keep the first and second gaskets out of complementary engagement with each other; wherein the hollow needle is axially movable relatively toward the first gasket to penetrate through the first gasket to cause the first containing space and the second containing space to communicate with each other through the hollow needle; wherein the first gasket and the second gasket are rotatable relative to each other and axially movable toward each other, whereby the first and second engagement portions (12, 13) are brought into complementary engagement with each other to reduce a volume of the second containing space. an outer cylinder; a hollow needle provided in the outer cylinder as extending axially and having a cutting edge at its front end; a first gasket provided in the outer cylinder on a front side of the hollow needle; and a second gasket provided in the outer cylinder on a front side of the first gasket; wherein a first space containing a liquid is defined in the outer cylinder on a rear side of the first gasket, and communicates with a rear end of the hollow needle, the syringe further comprising: non-rotatively and complementarily shaped first and second engagement portions (12, 13) respectively provided at a front end of the first gasket and a rear end of the second gasket; and characterized in that it further comprises a second space containing a drug to be mixed with the liquid is defined between the first gasket and the second gasket in the outer cylinder with a front end face of the first engagement portion and a rear end face of the second engagement portion abutting against each other to keep the first and second gaskets out of complementary engagement with each other; wherein the hollow needle is axially movable relatively toward the first gasket to penetrate through the first gasket to cause the first containing space and the second containing space to communicate with each other through the hollow needle; wherein the first gasket and the second gasket are rotatable relative to each other and axially movable toward each other, whereby the first and second engagement portions (12, 13) are brought into complementary engagement with each other to reduce a volume of the second containing space. non-rotatively and complementarily shaped first and second engagement portions (12, 13) respectively provided at a front end of the first gasket and a rear end of the second gasket; and characterized in that it further comprises a second space containing a drug to be mixed with the liquid is defined between the first gasket and the second gasket in the outer cylinder with a front end face of the first engagement portion and a rear end face of the second engagement portion abutting against each other to keep the first and second gaskets out of complementary engagement with each other; wherein the hollow needle is axially movable relatively toward the first gasket to penetrate through the first gasket to cause the first containing space and the second containing space to communicate with each other through the hollow needle; wherein the first gasket and the second gasket are rotatable relative to each other and axially movable toward each other, whereby the first and second engagement portions (12, 13) are brought into complementary engagement with each other to reduce a volume of the second containing space.