NOVELTY - An imaging plane is formed above a float (12) of radius (R) that floats on the surface of liquid fuel (11). A sunshade (16) is provided between the imaging plane and float. The concentration of the liquid fuel is calculated by a predetermined formula including parameters such as mass, volume, height and radius of the float.    USE - Used for detecting the concentration of liquid fuel in a liquid fuel cell such as an alkaline fuel cell, phosphorous acid fuel cell, proton exchange membrane fuel cell, molten carbonate fuel cell, solid oxide fuel cell, etc.    ADVANTAGE - The hardware cost for concentration detection of liquid fuel is reduced. The concentration detection device can be manufactured easily in mass production at low cost.    DETAILED DESCRIPTION - The radius of a reference circle of the float is r. The angle formed by one point and the center of the reference circle is alpha1. A hole (160) of the sunshade and imaging plane are arranged so that figures of reference and intersection circles are respectively projected on imaging plane through hole to form another reference and intersection circles that satisfy predetermined equations. The moving distance (S) and deflection angle (theta) of the float, and the vertical distance (F) between the hole and fuel surface are calculated based on the data of both reference circles. The u and v coordinate values for one point on another reference circle, and the x and y coordinate values for one point on another reference circle are calculated based on the phase angle (beta) of one point and center of reference circle. The angle (phi) between the imaging plane and fuel surface is calculated according to the equations of coordinate variable u and v, deflection angle and moving distance of float, vertical distance between hole and fuel surface and phase angle for reference circle. The angle (alpha) formed by one point and center of intersection circle is calculated according to the equations of coordinate variable x and y, deflection angle and moving distance of float, vertical distance between hole and fuel surface, phase angle for reference circle, angle between imaging plane and fuel surface. The height of float is calculated according to float radius and angle formed by one point and center of intersection circle. The concentration of the liquid fuel is calculated by a predetermined formula including parameters such as mass (M), volume (V), height (h) and radius of float.    An INDEPENDENT CLAIM is included for a concentration detection device.    DESCRIPTION OF DRAWING(S) - The figure shows a side view of the concentration detection device.    Liquid fuel (11)    Float (12)    Sunshade (16)    Hole (160)    Angle formed by one point and center of intersection circle (alpha)    Angle formed by one point and center of reference circle (alpha1)    Height of float (h)    Radius of reference circle (r)    Float radius (R)