The appearance mechanism of an electrochemical oscillation (previously called oscillation B), observed for Pt electrodes in H2O2-containing acidic electrolytes in a potential region of hydrogen evolution, has been investigated. Though it is reported that electrochemical oscillations in general appear in potential regions of negative differential resistance (NDR) or hidden NDR (HNDR), impedance analyses have shown that no NDR or HNDR is present in the potential region of oscillation B. Besides, oscillation B is observed only for Pt electrodes with atomically roughened surfaces, not for electrodes with atomically flat surfaces. A possible explanation is proposed by assuming the presence of small local "active" areas in the electrode surface, in which the H2O2 reduction is not prevented by a full coverage of under-potential deposited hydrogen, contrary to the other normal "nonactive" areas occupying the major part of the electrode surface. The appearance of oscillation B is reproduced by mathematical simulation based on the model, together with the consideration of electrical coupling between the active and nonactive areas as well as solution stirring by hydrogen-gas evolution. Oscillation B can be classified into a new category of oscillators, which may be called "coupled NDR" oscillators.