A reservoir tank comprising a reservoir body which reserves an operating liquid in an inner portion thereof formed to be in a hollow box shape, the reservoir tank being connected to a vehicle mounted apparatus for a fluid communication of the operating liquid with the vehicle mounted apparatus, wherein the reservoir body includes: a tubular projecting portion extending outward from an outer wall portion; a first partition portion which divides the inner portion of the reservoir body into a first chamber (R1) which reserves the operating liquid therein and a second chamber (R2) which volume is smaller than the volume of the first chamber and being in communication with the tubular projecting portion; and a first communication passage positioned at a portion lower than a vertically changeable liquid surface of the operating liquid reserved in the first chamber in a vertical direction and provided at the first partition portion to allow a communication between the first chamber and the second chamber, wherein the second chamber (R2) and the tubular projecting portion are kept in a state in which the first chamber (R1) and the second chamber (R2) are filled with the operating liquid which is supplied thereto from the first chamber (R1) via the first communication passage , and in the horizontal direction, the first chamber (R1) is adjacent to the second chamber (R2), and wherein the tubular projecting portion is provided at an upper portion higher than a position of the first communication passage in the vertical direction, characterized in that , in the horizontal direction, an opening area of the first communication passage at a first chamber (R1) side is set to be larger than an opening area of the first communication passage at a second chamber (R2) side. a tubular projecting portion extending outward from an outer wall portion; a first partition portion which divides the inner portion of the reservoir body into a first chamber (R1) which reserves the operating liquid therein and a second chamber (R2) which volume is smaller than the volume of the first chamber and being in communication with the tubular projecting portion; and a first communication passage positioned at a portion lower than a vertically changeable liquid surface of the operating liquid reserved in the first chamber in a vertical direction and provided at the first partition portion to allow a communication between the first chamber and the second chamber, wherein the second chamber (R2) and the tubular projecting portion are kept in a state in which the first chamber (R1) and the second chamber (R2) are filled with the operating liquid which is supplied thereto from the first chamber (R1) via the first communication passage , and in the horizontal direction, the first chamber (R1) is adjacent to the second chamber (R2), and wherein the tubular projecting portion is provided at an upper portion higher than a position of the first communication passage in the vertical direction, characterized in that , in the horizontal direction, an opening area of the first communication passage at a first chamber (R1) side is set to be larger than an opening area of the first communication passage at a second chamber (R2) side. <EOS>