PURPOSE: To prevent rapid wear of a level sensor body and to facilitate cleaning of the interior of a hopper.

CONSTITUTION: (1) During unmanned operation (automatic operation) of a concrete pump, a pin with a chain is pulled off from the pin hole of a bracket 10. An electrode (a level sensor body) 2 is rotated, in order, to a solid line position (a horizontal position) and a two-dot chain line position (a vertical position) around a pivotal support pin 5 and moved to the contact position of concrete. A pin 7 with a chain is then inserted in the pin hole of a bracket 6 and the electrode 2 is held in a vertical position. (2) During manned operation, the pin 7 with a chain is pulled off through the pin hole of the bracket 6 and the electrode 2 is then rotated, in order, to a two-dot chain line position (a vertical position) and a solid line position (a horizontal position) around the pivotal support pin 5 and moved to the non-contact position of concrete. A pin 11 with a chain is then inserted in the pin hole of the bracket 10 and the electrode 2 is held in a horizontal position.