We have conducted a paleomagnetic investigation of the Kozagawa Dike, which is part of the Kumano Acidic Rocks that are the product of a Middle Miocene volcano–plutonic complex cropping out on the Kii Peninsula of Southwest Japan. The granite porphyry facies of the dike yields stable remanent magnetization components. Precisely determined site-mean directions of characteristic components, as revealed by principal component analysis of stepwise alternating-field and thermal demagnetization results, form a tight cluster. The overall mean direction exhibits an approximately 40° clockwise declination shift from the south and a steep negative inclination (D = 217.3°, I = −59.8°, α95 = 2.8°; 10 sites). Rock magnetic analyses suggest that magnetite is the main stable component carrier. The overall mean direction is interpreted to be a geologically instantaneous record of the paleomagnetic field that was acquired in a relatively short time interval relative to paleomagnetic secular variations. The mean direction in geographic (in situ) coordinates obtained for the Kozagawa Dike is distinct from previously published data for laccolithic granite porphyry intrusions of the Kumano Acidic Rocks. This difference may result from a time lag during acquisition of remanent magnetization by the different units of the Kumano Acidic Rocks. The southwest and up paleomagnetic direction has previously been interpreted as representing clockwise rotation of Southwest Japan associated with back-arc opening of the Japan Sea. However, we contend that these directions actually reflect the transient capture of an extraordinary paleomagnetic field at ca 14.3 Ma, such as a transitional or excursional state, and are not of tectonic origin.