A seismic reflection survey was conducted in the northwestern Pacific to investigate subsurface structure of small volcanoes considered to be formed by the newly-discovered “petit-spot” intra-plate volcanism. In contrast with an acoustically transparent sedimentary layer in the ambient northwestern Pacific, sedimentary layers are acoustically opaque with interbedded strong reflections beneath the volcanoes. These reflections are possibly caused by inhomogeneous structure due to intrusion of central vent. Subhorizontal strong reflections at base of the sedimentary layer, identified in the vicinity of volcanoes, are probably reflections from sills of dense lavas flowing out within the sedimentary layer. These volcanoes could be monogenetic volcanoes produced by small amount of magma intrusion,<1 km³ in volume. Although such small volcanoes have never been discovered unless through a high-resolution bathymetric mapping, the “invisible” intra-plate volcanism affects successive evolution/modification of the crustal architecture of old-age oceanic plates.