A rotary toothbrush including a housing, a brush that is rotatably attached to the housing, and a motor, which is installed inside the housing and rotates the brush, is provided. The brush includes a first end that is rotatably attached to the housing, and a second end that is opposite to the first end. The diameter of the brush increases toward the second end. The diameter of the brush linearly increases from the first end to the second end. Alternatively, the diameter of the brush exponentially increases from the first end to the second end. The brush may further comprise a portion that has a constant diameter near the second end. A shaft connects the brush and the motor. The rotational speed of the shaft is adjustable. The rotary toothbrush further includes a cover that partially covers the brush in an arc.