A lamp device includes: a battery housing receiving a battery set and a circuit board; a lamp casing enclosing and connected movably to the battery housing such that the lamp casing is movable relative to the battery housing in an axial direction; an LED lighting unit mounted in and connected fixedly to the lamp casing, and disposed spacedly above the circuit board; and a power switch mounted on the circuit board and connected electrically between the LED lighting unit and the battery set. The power switch is actuated in response to movement of the LED lighting unit toward the circuit board by press of the lamp casing to switch from a current one of power-on and power-off states to the other one of the power-on and power-off states.