PROBLEM TO BE SOLVED: To provide an apparatus and method that can reduce man-power work such as divers (diving person) to 10% or less in the cutting-processing work such as chipping processing of an object plane such as a concrete wall surface in water.SOLUTION: An apparatus for cutting an object plane in water of the present invention includes: a fixing member (50a) for holding a suspended counter weight (CW) so that a pressing force may be given to the object plane (D) in an unloaded condition by a crane (CR); a cutting processing machine (51) for performing cutting-processing of the object plane (D) detachably fixed to the fixing member (50a); and one or two or more jet nozzles (JN) fixed through a shielding plate (52) provided in the fixing member (50a). The jet nozzle (JN) is characterized in that high-pressure water is supplied from a water jet pump (water compressor) (WP) installed ashore through a tube (WT), and a thrust force (TF) is generated in the direction apart from the object plane (D).
