A valve device for a fuel tank, comprising: an upper-portion (Ta) including a first valve opening and a second valve opening for communicating inside and outside of a fuel tank; a lower-portion assembled with the upper-portion (Ta), and including a first valve chamber formed below the first valve opening, a second valve chamber formed below the second valve opening and partitioned from the first valve chamber, a partition wall separating the first valve chamber and the second valve chamber, a first support portion having a passage portion for a fluid so that the first valve chamber has an open end below the support portion, and a second support portion having another passage portion for a fluid; a first float disposed inside the first valve chamber and supported on the first support portion to be movable up and down; and a second float disposed inside the second valve chamber and supported on the second support portion to be movable up and down, wherein the first valve chamber and the second valve chamber are assembled so that the lower-portion including the first support portion for the first valve and the second support portion for the second float is fitted to the upper-portion (Ta), characterized in that the upper-portion (Ta) forming the first valve chamber includes a fitted portion having an oval shape in a horizontal cross section, and the lower-portion fitting into the fitted portion has a fitting portion in a horizontal cross section substantially matching with the oval shape of the fitted portion when the upper-portion and the lower-portion fit together, and, the fitted portion and the fitting portion fit together in such a way so as to position one of the fitted portion and the fitting portion inside another of the fitted portion and the fitting portion. an upper-portion (Ta) including a first valve opening and a second valve opening for communicating inside and outside of a fuel tank; a lower-portion assembled with the upper-portion (Ta), and including a first valve chamber formed below the first valve opening, a second valve chamber formed below the second valve opening and partitioned from the first valve chamber, a partition wall separating the first valve chamber and the second valve chamber, a first support portion having a passage portion for a fluid so that the first valve chamber has an open end below the support portion, and a second support portion having another passage portion for a fluid; a first float disposed inside the first valve chamber and supported on the first support portion to be movable up and down; and a second float disposed inside the second valve chamber and supported on the second support portion to be movable up and down, wherein the first valve chamber and the second valve chamber are assembled so that the lower-portion including the first support portion for the first valve and the second support portion for the second float is fitted to the upper-portion (Ta), characterized in that the upper-portion (Ta) forming the first valve chamber includes a fitted portion having an oval shape in a horizontal cross section, and the lower-portion fitting into the fitted portion has a fitting portion in a horizontal cross section substantially matching with the oval shape of the fitted portion when the upper-portion and the lower-portion fit together, and, the fitted portion and the fitting portion fit together in such a way so as to position one of the fitted portion and the fitting portion inside another of the fitted portion and the fitting portion.