A test method for a spark plug ceramic insulator includes placing a first electrode in an inner hole of the ceramic insulator and placing a second electrode on an outer peripheral side of the ceramic insulator, developing a defect in the ceramic insulator by the application of a first voltage onto the ceramic insulator between the first and second electrodes and detecting the defect in the ceramic insulator by the application of a second voltage, which is lower than a flashover voltage that causes a flashover of the ceramic insulator, onto the ceramic insulator between the first and second electrodes.