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. However, by removing the controller from the pump casing and the external power connector and by directly connecting the external power connector to the power connector of the pump, the pump may be converted to a non-automatic operation mode.