Provided are a machining device (10), a machining unit, and a machining method that irradiate a workpiece (8) with a laser beam to perform cutting or boring machining of the workpiece (8). The invention has a laser output device (12), a guiding optical system (14) that guides a laser beam, and an irradiating head (16) that guides a laser beam and irradiates the workpiece (8) with the laser beam. The irradiating head (16) integrally rotates a first prism (52) and a second prism (54) with a rotation mechanism, thereby rotating a light path of the laser beam around a rotational axis of the rotation mechanism and irradiating the workpiece (8) while rotating the position of irradiation to the workpiece. A control device (22) calculates an allowable rotational frequency range of the laser beam on the basis of the relationship between an allowable thickness of a remelted layer of the workpiece (8) and a rotational frequency, or the relationship between an allowable thickness of an oxidization layer of the workpiece and the rotational frequency, determines a rotational frequency included in the allowable rotational frequency range as the rotational frequency of the rotation mechanism, and rotates the rotation mechanism at the determined rotational frequency, thereby enabling high-precision machining to be performed with a simple configuration.