A friction stir spot welding device (50A, 50B, 50C, 50D) that welds an object to be welded by partial stirring of a rotating tool, the device (50A, 50B, 50C, 50D) comprising: a cylindrical pin member as the rotating tool, the pin member configured to rotate about an axis and be advanceable and retractable in the axial direction; a tubular shoulder member configured to surround the pin member, rotate coaxially with the pin member, and be advanceable and retractable in the axial direction; a tool driving section configured to cause each of the pin member and the shoulder member to advance and retract along the axis (X v); and a tool driving control section configured to control the action of the tool driving section to advance and retract the pin member or the shoulder member to bring a contact surface of the pin member and a contact surface of the shoulder member in contact with a front surface of the object to be welded, wherein the tool driving control section controls the tool driving section to adjust a press-fit depth P p of the pin member press-fitted from a front surface of the object to be welded, and/or a press-fit depth P s of the shoulder member press-fitted from the front surface of the object to be welded, wherein Pp and Ps indicate a press-fit depth from a preset reference point, and wherein P P respectively P S is defined to be negative when a front end surface of the pin member respectively shoulder member is away from the surface of the object to be welded and P P respectively P S is defined to be positive when the front end surface of the pin member respectively shoulder member is press-fitted into the object to be welded such that when a front end surface of the pin member is away from the surface of the object to be welded (i. e. P p < 0), P s becomes positive and when a front end surface of the pin member is press-fitted into the object to be welded ( (i.e. P p > 0), P s becomes negative, and an absolute value of a tool average position T x defined by the following equation: A p ⋅ P p + A s ⋅ P s = T x, where A p is a cross-section area of a front end surface of the pin member, A s is a cross-section area of a front end surface of the shoulder member, is substantially 0. a cylindrical pin member as the rotating tool, the pin member configured to rotate about an axis and be advanceable and retractable in the axial direction; a tubular shoulder member configured to surround the pin member, rotate coaxially with the pin member, and be advanceable and retractable in the axial direction; a tool driving section configured to cause each of the pin member and the shoulder member to advance and retract along the axis (X v); and a tool driving control section configured to control the action of the tool driving section to advance and retract the pin member or the shoulder member to bring a contact surface of the pin member and a contact surface of the shoulder member in contact with a front surface of the object to be welded, wherein the tool driving control section controls the tool driving section to adjust a press-fit depth P p of the pin member press-fitted from a front surface of the object to be welded, and/or a press-fit depth P s of the shoulder member press-fitted from the front surface of the object to be welded, wherein Pp and Ps indicate a press-fit depth from a preset reference point, and wherein P P respectively P S is defined to be negative when a front end surface of the pin member respectively shoulder member is away from the surface of the object to be welded and P P respectively P S is defined to be positive when the front end surface of the pin member respectively shoulder member is press-fitted into the object to be welded such that when a front end surface of the pin member is away from the surface of the object to be welded (i. e. P p < 0), P s becomes positive and when a front end surface of the pin member is press-fitted into the object to be welded ( (i.e. P p > 0), P s becomes negative, and an absolute value of a tool average position T x defined by the following equation: A p ⋅ P p + A s ⋅ P s = T x, where A p is a cross-section area of a front end surface of the pin member, A s is a cross-section area of a front end surface of the shoulder member, is substantially 0.