A tool driving section of a friction stir spot welding device is configured to advance and retract each of a pin member and a shoulder member. A tool driving control section is configured to control the tool driving section such that an absolute value of a tool average position Tx defined by a following equation: Ap·Pp+As·Ps=Tx, where Ap is a cross-section area of a front end surface of the pin member, As is a cross-section area of a front end surface of the shoulder member, Pp is a press-fit depth of the pin member press-fitted from a front surface of an object to be welded, and Ps is a press-fit depth of the shoulder member press-fitted from the front surface of the object to be welded, is small.