An underwater detection apparatus, comprising: a transmission transducer configured to transmit a transmission wave; a reception transducer comprising a plurality of reception elements, each of the reception elements generating a reception signal based on a reflection wave generated by reflection of the transmission wave on an underwater target; a volume data generation module configured to generate volume data based on the reception signal, the volume data including echo intensity data at each reference position within a detection range; and a group generation module configured to group the echo intensity data of the volume data into a plurality of groups, by setting a given threshold to the volume data; extracting data from volume data with an echo intensity above the given threshold; generating isosurfaces based on the extracted data; determining a connection between the generated isosurfaces; and setting isosurfaces for which the connection is confirmed, as the same group; characterised by: a representative echo intensity calculation module configured to calculate a representative echo intensity of a first group from the plurality of groups based on a plurality of echo intensity data contained within the connected isosurfaces of the first group; and an image data generation module configured to set a property of the connected isosurfaces of the first group based on the representative echo intensity of the first group. a transmission transducer configured to transmit a transmission wave; a reception transducer comprising a plurality of reception elements, each of the reception elements generating a reception signal based on a reflection wave generated by reflection of the transmission wave on an underwater target; a volume data generation module configured to generate volume data based on the reception signal, the volume data including echo intensity data at each reference position within a detection range; and a group generation module configured to group the echo intensity data of the volume data into a plurality of groups, by setting a given threshold to the volume data; extracting data from volume data with an echo intensity above the given threshold; generating isosurfaces based on the extracted data; determining a connection between the generated isosurfaces; and setting isosurfaces for which the connection is confirmed, as the same group; characterised by: a representative echo intensity calculation module configured to calculate a representative echo intensity of a first group from the plurality of groups based on a plurality of echo intensity data contained within the connected isosurfaces of the first group; and an image data generation module configured to set a property of the connected isosurfaces of the first group based on the representative echo intensity of the first group. a representative echo intensity calculation module configured to calculate a representative echo intensity of a first group from the plurality of groups based on a plurality of echo intensity data contained within the connected isosurfaces of the first group; and an image data generation module configured to set a property of the connected isosurfaces of the first group based on the representative echo intensity of the first group.