A laundry treating apparatus comprising: a tub providing a space for containing water; a drum rotatably provided in the tub to provide a space for storing clothes; an agitator rotatably provided in the drum; a stator provided outside the tub to generate a rotating magnetic field; a rotor provided outside the tub, being configured to be rotated by the rotating magnetic field; a drum-rotating shaft passing through the tub, wherein the drum-rotating shaft comprises one end fixed to the drum and the other end as a free end located outside the tub; an agitator-rotating shaft passing through the drum-rotating shaft in a longitudinal direction thereof, wherein the agitator-rotating shaft comprises one end fixed to the rotor and the other end fixed to the agitator; a first serration provided on the one end of the agitator-rotating shaft; a second serration provided on the free end of the drum-rotating shaft; a fixed body having a cylindrical shape with an open top surface, being fixed between the tub and the rotor, wherein the fixed body has a first through-hole provided at a bottom surface thereof to allow the agitator-rotating shaft to pass therethrough; a support body having a cylindrical shape with an open top surface, being rotatable within the fixed body, wherein the support body has a second through-hole provided at a bottom surface thereof to allow the agitator-rotating shaft to pass therethrough; and a body-driving system configured to rotate the support body within the fixed body, characterized by further comprising: a shaft-joint rotatably provided on the support body to reciprocate between a first and a second point as the support body changes its vertical level, wherein the fixed body and the support body are arranged to form a convert mechanism configured to convert a rotational motion of the support body into a linear reciprocating motion of the support body with respect to the fixed body; and wherein the first point of the shaft-joint is set such that the first serration and the second serration are engaged with each other, and wherein the second point of the shaft-joint is set such that the first serration and the second serration are disengaged from each other. a tub providing a space for containing water; a drum rotatably provided in the tub to provide a space for storing clothes; an agitator rotatably provided in the drum; a stator provided outside the tub to generate a rotating magnetic field; a rotor provided outside the tub, being configured to be rotated by the rotating magnetic field; a drum-rotating shaft passing through the tub, wherein the drum-rotating shaft comprises one end fixed to the drum and the other end as a free end located outside the tub; an agitator-rotating shaft passing through the drum-rotating shaft in a longitudinal direction thereof, wherein the agitator-rotating shaft comprises one end fixed to the rotor and the other end fixed to the agitator; a first serration provided on the one end of the agitator-rotating shaft; a second serration provided on the free end of the drum-rotating shaft; a fixed body having a cylindrical shape with an open top surface, being fixed between the tub and the rotor, wherein the fixed body has a first through-hole provided at a bottom surface thereof to allow the agitator-rotating shaft to pass therethrough; a support body having a cylindrical shape with an open top surface, being rotatable within the fixed body, wherein the support body has a second through-hole provided at a bottom surface thereof to allow the agitator-rotating shaft to pass therethrough; and a body-driving system configured to rotate the support body within the fixed body, characterized by further comprising: a shaft-joint rotatably provided on the support body to reciprocate between a first and a second point as the support body changes its vertical level, wherein the fixed body and the support body are arranged to form a convert mechanism configured to convert a rotational motion of the support body into a linear reciprocating motion of the support body with respect to the fixed body; and wherein the first point of the shaft-joint is set such that the first serration and the second serration are engaged with each other, and wherein the second point of the shaft-joint is set such that the first serration and the second serration are disengaged from each other. a shaft-joint rotatably provided on the support body to reciprocate between a first and a second point as the support body changes its vertical level, wherein the fixed body and the support body are arranged to form a convert mechanism configured to convert a rotational motion of the support body into a linear reciprocating motion of the support body with respect to the fixed body; and wherein the first point of the shaft-joint is set such that the first serration and the second serration are engaged with each other, and wherein the second point of the shaft-joint is set such that the first serration and the second serration are disengaged from each other.