Abstract: A controller for an underwater pump which operates to start and stop the pump in response to a water level detected by a water level sensor is disclosed. The controller (10) is constituted as a separate component which comprises a watertight case (11), a first connector (13) provided at a first predetermined position of the case for installation on a power connector (12) provided at a pump casing (51), a second connector (14) provided at a second predetermined position on the case, on which an external power connector (15) is to be installed, electronic parts and units (18, 19, 20) constituting a control circuit installed in the case for starting and stopping the pump based on signals output from the water level detection sensor (S.sub.1, S.sub.2, S.sub.3), and a cable (22) watertightly derived from the case externally for electrically connecting the control circuit and the water level sensor. The pump may be operated in an automatic operation mode.