An underwater detection apparatus (1, 1A, 1C, 1D, 1E), comprising: a transmission transducer (11, 12) configured to transmit a transmission wave within a given transmission fan-shaped space (T1, T2, T1E), the transmission fan-shaped space (T1, T2, T1E) having a first transmission width (Tθ1, Tθ3) in a given first plane and a second transmission width (Tθ2, Tθ4) in a second plane perpendicular to the first plane; a reception transducer (13, 14) configured to receive a reflection wave of the transmission wave within a given reception fan-shaped space (R1, R2, R1E), the reception fan-shaped space (R1, R2, R1E) having a first reception width (Rθ1, Rθ3) in the first plane and a second reception width (Rθ2, Rθ4) in the second plane, the second reception width (Rθ2, Rθ4) being narrower than the second transmission width (Tθ2, Tθ4); and in the second plane, one of a pair of edges of the transmission fan shaped space (T1, T2, T1E) being within the reception fan shaped space (R1, R2, R1E); a motor configured to rotate the transmission fan-shaped space (T1, T2, T1E) and the reception fan-shaped space (R1, R2, R1E); and a controller configured to control the motor; characterized in that: the controller controls the motor to rotate at a given first speed when the transmission transducer and the reception transducer perform an underwater detection, and at a second speed faster than the first speed when the underwater detection is not performed; wherein the controller controls the motor to rotate in a given first direction in both cases when the underwater detection is performed and when the underwater detection is not performed. a transmission transducer (11, 12) configured to transmit a transmission wave within a given transmission fan-shaped space (T1, T2, T1E), the transmission fan-shaped space (T1, T2, T1E) having a first transmission width (Tθ1, Tθ3) in a given first plane and a second transmission width (Tθ2, Tθ4) in a second plane perpendicular to the first plane; a reception transducer (13, 14) configured to receive a reflection wave of the transmission wave within a given reception fan-shaped space (R1, R2, R1E), the reception fan-shaped space (R1, R2, R1E) having a first reception width (Rθ1, Rθ3) in the first plane and a second reception width (Rθ2, Rθ4) in the second plane, the second reception width (Rθ2, Rθ4) being narrower than the second transmission width (Tθ2, Tθ4); and in the second plane, one of a pair of edges of the transmission fan shaped space (T1, T2, T1E) being within the reception fan shaped space (R1, R2, R1E); a motor configured to rotate the transmission fan-shaped space (T1, T2, T1E) and the reception fan-shaped space (R1, R2, R1E); and a controller configured to control the motor; characterized in that: the controller controls the motor to rotate at a given first speed when the transmission transducer and the reception transducer perform an underwater detection, and at a second speed faster than the first speed when the underwater detection is not performed; wherein the controller controls the motor to rotate in a given first direction in both cases when the underwater detection is performed and when the underwater detection is not performed. the controller controls the motor to rotate at a given first speed when the transmission transducer and the reception transducer perform an underwater detection, and at a second speed faster than the first speed when the underwater detection is not performed; wherein the controller controls the motor to rotate in a given first direction in both cases when the underwater detection is performed and when the underwater detection is not performed.