A method for detecting leak from a fuel vapor treating apparatus defining a first area including a fuel tank and a second area including an adsorbent canister has hermetically closing the first area, measuring internal pressure of the first area, comparing an absolute value of differential pressure between the internal pressure of the first area and the atmospheric pressure with a predetermined value, measuring the internal pressure of the first area in a case that the absolute value is equal to or higher than the predetermined value in order to check for leaks from the first area based on changes in the internal pressure of the first area, fluidly communicating the first area with the second area in order to equilibrate internal pressures of the first area and the second area, hermetically closing the second area, and measuring the internal pressure of the second area in order to check for leaks from the second area based on changes in the internal pressure of the second area.