A Group III and V element compound semiconductor such as gallium arsenide is formed on a semiconductor wafer by so-called MOCVD. A first pair of convex portions, a second pair of convex portions and crossing portion are formed from such compound semiconductor by an etching using a predetermined etching substance so that one convex portion of each pair is opposite to the other convex portion thereof and that a same crystalline surface of the crossing portion is exposed at all points where the first pair of convex portions crosses the second pair of convex portions. A pair of input terminals and a pair of output terminals are electrically connected to each convex portion of the first pair and the second pair, respectively so as to input electric current to each convex portion of the first pair and to output voltage generated in response to a magnetic field strength in such compound semiconductor. Accordingly, the occurrence of the unbalanced voltage is prevented because of the geometrical balance of two pair convex portions and crossing portion.