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We have developed a near-field scanning optical microscope which has the capability of imaging in both constant-height and constant-gap scanning modes. This microscope uses an apertureless metallic probe. The constant-gap position of the probe is controlled by regulation of the tunneling-electron current. After obtaining a constant-gap signal under the regulation at a pixel, the constant-height signal is measured at a probe height, where is the same height at every pixel. By repeating the measurement at each pixel, we got both constant-gap and constant-height images. The result shows that the two imaging modes provide different informations. The constant-gap image has higher contrast than the constant-height one. Artifacts in near-field images measured by the apertureless type microscope are also discussed.