A sigmoidoscope comprising: a main body having an optic head having a channel extending through the optic head; and a positioning recess disposed at a middle portion of the optic head; a handle connected to the optic head and having a chamber communicating with the channel; and an illuminating assembly having a circuit board disposed in the positioning recess and having an annular outline; and a central hole aligned with the channel; multiple light emitting elements (22,22A) annularly mounted to the circuit board at intervals and electrically connected to the circuit board; two wires electrically connected to the circuit board; two electrodes respectively and electrically connected to the two wires; and at least one battery electrically connecting the two electrodes, the sigmoidoscope being further characterized by having a tact switch mounted to the circuit board, electrically connected to the circuit board and by having a pushing button extending over a rim of the central hole and being able to be pressed by an inserting tube of a speculum assembly that is inserted into the central hole. a main body having an optic head having a channel extending through the optic head; and a positioning recess disposed at a middle portion of the optic head; a handle connected to the optic head and having a chamber communicating with the channel; and an optic head having a channel extending through the optic head; and a positioning recess disposed at a middle portion of the optic head; a channel extending through the optic head; and a positioning recess disposed at a middle portion of the optic head; a handle connected to the optic head and having a chamber communicating with the channel; and an illuminating assembly having a circuit board disposed in the positioning recess and having an annular outline; and a central hole aligned with the channel; multiple light emitting elements (22,22A) annularly mounted to the circuit board at intervals and electrically connected to the circuit board; two wires electrically connected to the circuit board; two electrodes respectively and electrically connected to the two wires; and at least one battery electrically connecting the two electrodes, the sigmoidoscope being further characterized by having a tact switch mounted to the circuit board, electrically connected to the circuit board and by having a pushing button extending over a rim of the central hole and being able to be pressed by an inserting tube of a speculum assembly that is inserted into the central hole. a circuit board disposed in the positioning recess and having an annular outline; and a central hole aligned with the channel; an annular outline; and a central hole aligned with the channel; multiple light emitting elements (22,22A) annularly mounted to the circuit board at intervals and electrically connected to the circuit board; two wires electrically connected to the circuit board; two electrodes respectively and electrically connected to the two wires; and at least one battery electrically connecting the two electrodes, the sigmoidoscope being further characterized by having a tact switch mounted to the circuit board, electrically connected to the circuit board and by having a pushing button extending over a rim of the central hole and being able to be pressed by an inserting tube of a speculum assembly that is inserted into the central hole.