A handheld sanding device comprising: a power generation unit configured to generate rotating force for sanding operation; a power transmission unit configured to be coupled to the power generation unit, and to transmit the rotating force generated by the power generation unit; a sanding unit configured to be rotated by the rotating force, transmitted through the power transmission unit, in order to sand a sanding target surface; and a shaft coupling unit configured to connect the power transmission unit and the sanding unit to each other so that the sanding unit is freely inclined based on the power transmission unit, wherein the shaft coupling unit comprises: a fixed shaft portion comprising a fixed shaft configured to be coupled to the power transmission unit and to be rotated by the rotating force transmitted through the power transmission unit and a separation prevention ball configured to extend from the fixed shaft; and a rotating shaft portion comprising an autonomous rotation block configured to be provided with a ball movement cavity into and on which the separation prevention ball is inserted and supported so that the separation prevention ball is reciprocated or inclined and an autonomous rotation shaft configured to extend from the autonomous rotation block and to be coupled to the sanding unit, and characterized in that an outer circumferential surface of the separation prevention ball is provided with a flat surface-shaped first stop surface configured to intersect a normal to an imaginary sphere, including the separation prevention ball; an inner surface of the ball movement cavity is provided with a flat surface-shaped second stop surface configured to intersect a normal to an imaginary sphere, including the ball movement cavity, to correspond to the first stop surface; and a gap is formed between the separation prevention ball and the ball movement cavity so that the separation prevention ball is reciprocated and inclined within the ball movement cavity. a power generation unit configured to generate rotating force for sanding operation; a power transmission unit configured to be coupled to the power generation unit, and to transmit the rotating force generated by the power generation unit; a sanding unit configured to be rotated by the rotating force, transmitted through the power transmission unit, in order to sand a sanding target surface; and a shaft coupling unit configured to connect the power transmission unit and the sanding unit to each other so that the sanding unit is freely inclined based on the power transmission unit, wherein the shaft coupling unit comprises: a fixed shaft portion comprising a fixed shaft configured to be coupled to the power transmission unit and to be rotated by the rotating force transmitted through the power transmission unit and a separation prevention ball configured to extend from the fixed shaft; and a rotating shaft portion comprising an autonomous rotation block configured to be provided with a ball movement cavity into and on which the separation prevention ball is inserted and supported so that the separation prevention ball is reciprocated or inclined and an autonomous rotation shaft configured to extend from the autonomous rotation block and to be coupled to the sanding unit, and characterized in that an outer circumferential surface of the separation prevention ball is provided with a flat surface-shaped first stop surface configured to intersect a normal to an imaginary sphere, including the separation prevention ball; an inner surface of the ball movement cavity is provided with a flat surface-shaped second stop surface configured to intersect a normal to an imaginary sphere, including the ball movement cavity, to correspond to the first stop surface; and a gap is formed between the separation prevention ball and the ball movement cavity so that the separation prevention ball is reciprocated and inclined within the ball movement cavity. a fixed shaft portion comprising a fixed shaft configured to be coupled to the power transmission unit and to be rotated by the rotating force transmitted through the power transmission unit and a separation prevention ball configured to extend from the fixed shaft; and a rotating shaft portion comprising an autonomous rotation block configured to be provided with a ball movement cavity into and on which the separation prevention ball is inserted and supported so that the separation prevention ball is reciprocated or inclined and an autonomous rotation shaft configured to extend from the autonomous rotation block and to be coupled to the sanding unit, and characterized in that an outer circumferential surface of the separation prevention ball is provided with a flat surface-shaped first stop surface configured to intersect a normal to an imaginary sphere, including the separation prevention ball; an inner surface of the ball movement cavity is provided with a flat surface-shaped second stop surface configured to intersect a normal to an imaginary sphere, including the ball movement cavity, to correspond to the first stop surface; and a gap is formed between the separation prevention ball and the ball movement cavity so that the separation prevention ball is reciprocated and inclined within the ball movement cavity.