In February, 1888, Heinrich R. Hertz published an experiment which illustrated the existance of electromagnetic waves in the air. It also proved that their velocity of propagation is the same order as that of the light velocity. The result was soon regarded by the "Maxwellian physicists" as the crucial experimental proof of Maxwell's electromagnetic theory of light. Since then, most of continental physicists, who had been under the influence of action-at-a-distance theory, evaluated Hertz's experiment in the same way and accepted Maxwell's electromagnetic field theory. However, Hertz did not interpreted his result like that; in his opinion, what was proved by his expriment was just the electromnagnetic action of dielectric materials, not Maxwell's theory, because the propagating velocity was differnt from the value that Maxwell's theory predicted. In this paper, I will show three points : first, the experiment in 1888 was not intended to confirm Maxwell's theory, but to investigate Helmholtz's ideas on the electromagnetic action of dieleclectric in the case of open circuit; second, as experiments went on, at least by 1887, Hertz came to deny action-at-a-distance in electromagntic phenomena, but he still had Helmhotz's physical concepts which were diffrent from Maxwell's; finally, the interpretation that Hertz confirmed Maxwell's theory was made by the "Maxwellians" who did not have any practical means to support Maxwell's theory which they believed as true.