Abstract: An instrument for measuring dimensions of a can seam portion including a criterion measuring pin, a linear gauge provided with a measuring spindle which is so supported as to be movable along a straight line L in the direction of approaching said criterion measuring pin and in the direction of leaving said criterion measuring pin, a can cap supporting block, and a can body supporting block. The can cap supporting block holds the can such that the direction of measuring can seaming thickness T coincides with the straight line L so that the inner face of the can seam portion contacts the criterion measuring pin. The can body supporting block holds the can such that the direction for measuring the can seaming width W is identical with the straight line L and the lower face so that the can seam portion contacts the criterion measuring pin.