A separator for an electrochemical device, the separator comprising: a porous polymer substrate including a polymer resin; and a porous coating layer formed on at least one surface of both surfaces of the porous polymer substrate; wherein the porous coating layer comprises a) plate-like inorganic particles and b) spherical inorganic particles; and wherein one of the following conditions (i) and (ii) is fulfilled: (i) the porous coating layer shows a step-wise or successive increase in the content of inorganic particles a) from the bottom close to the porous polymer substrate to the top, when viewed from the thickness direction of the porous coating layer, and shows a step-wise or successive decrease in the content of inorganic particles b) from the bottom close to the porous polymer substrate to the top, when viewed from the thickness direction of the porous coating layer; or (ii) the porous coating layer comprises the inorganic particles b) at a higher content, as compared to the inorganic particles a) in the portion from the bottom close to the porous polymer substrate to 10% of the thickness, and comprises the inorganic particles a) in an amount of 50 wt% or more based on 100 wt% of the combined weight of the inorganic particles a) and b) in the portion from the surface of the porous coating layer for facing to an electrode to 10% of the thickness, when viewed from the thickness direction. a porous polymer substrate including a polymer resin; and a porous coating layer formed on at least one surface of both surfaces of the porous polymer substrate; wherein the porous coating layer comprises a) plate-like inorganic particles and b) spherical inorganic particles; and wherein one of the following conditions (i) and (ii) is fulfilled: (i) the porous coating layer shows a step-wise or successive increase in the content of inorganic particles a) from the bottom close to the porous polymer substrate to the top, when viewed from the thickness direction of the porous coating layer, and shows a step-wise or successive decrease in the content of inorganic particles b) from the bottom close to the porous polymer substrate to the top, when viewed from the thickness direction of the porous coating layer; or (ii) the porous coating layer comprises the inorganic particles b) at a higher content, as compared to the inorganic particles a) in the portion from the bottom close to the porous polymer substrate to 10% of the thickness, and comprises the inorganic particles a) in an amount of 50 wt% or more based on 100 wt% of the combined weight of the inorganic particles a) and b) in the portion from the surface of the porous coating layer for facing to an electrode to 10% of the thickness, when viewed from the thickness direction. (i) the porous coating layer shows a step-wise or successive increase in the content of inorganic particles a) from the bottom close to the porous polymer substrate to the top, when viewed from the thickness direction of the porous coating layer, and shows a step-wise or successive decrease in the content of inorganic particles b) from the bottom close to the porous polymer substrate to the top, when viewed from the thickness direction of the porous coating layer; or (ii) the porous coating layer comprises the inorganic particles b) at a higher content, as compared to the inorganic particles a) in the portion from the bottom close to the porous polymer substrate to 10% of the thickness, and comprises the inorganic particles a) in an amount of 50 wt% or more based on 100 wt% of the combined weight of the inorganic particles a) and b) in the portion from the surface of the porous coating layer for facing to an electrode to 10% of the thickness, when viewed from the thickness direction.