A dissimilar metal welding method of overlapping a first metal material and a second metal material and performing friction stir spot welding of the first metal material and the second metal material, the second metal material being composed of a metal having a lower melting point than a melting point of a metal composing the first metal material, the dissimilar metal welding method using a welding device having a clamp, a cylindrical tool body supported by an upper-side member of the clamp in a rotatable and axially movable manner around and along a central axis line of the tool body and a column-shaped pin member inserted into an inner hole of the tool body in a rotatable and axially movable manner around and along the central axis line of the tool body, the dissimilar metal welding method comprising: a preparing step of causing a distal end surface of the pin member to abut against a predetermined position on a non-overlapping surface of the second metal material, by performing driving such that the distal end surface of the pin member is moved to the second metal material while the distal end surface of the pin member is rotated; and a working step of continuing the rotation and movement of the pin member, and performing driving such that a distal end surface of the tool body is moved in a direction away from the second metal material while the tool body is rotated, wherein: in the working step, a through-hole is formed in the second metal material by the pin member, metal of the second metal material that plastically flows is collected in an annular space formed by an outer circumference of the pin member, the distal end surface of the tool body and an inner circumference of the upper-side member of the clamp, and then a distal end of the pin member is dug into the first metal material to a predetermined depth position in the first metal material; and the working step further being characterized by movements of the pin member and the tool body being stopped when at least one of a change amount of an electric current that is applied to a drive source for rotating the pin member and a change amount of an electric current that is applied to a drive source for axially moving the pin member becomes larger than a predetermined threshold in the working step. a preparing step of causing a distal end surface of the pin member to abut against a predetermined position on a non-overlapping surface of the second metal material, by performing driving such that the distal end surface of the pin member is moved to the second metal material while the distal end surface of the pin member is rotated; and a working step of continuing the rotation and movement of the pin member, and performing driving such that a distal end surface of the tool body is moved in a direction away from the second metal material while the tool body is rotated, wherein: in the working step, a through-hole is formed in the second metal material by the pin member, metal of the second metal material that plastically flows is collected in an annular space formed by an outer circumference of the pin member, the distal end surface of the tool body and an inner circumference of the upper-side member of the clamp, and then a distal end of the pin member is dug into the first metal material to a predetermined depth position in the first metal material; and the working step further being characterized by movements of the pin member and the tool body being stopped when at least one of a change amount of an electric current that is applied to a drive source for rotating the pin member and a change amount of an electric current that is applied to a drive source for axially moving the pin member becomes larger than a predetermined threshold in the working step. in the working step, a through-hole is formed in the second metal material by the pin member, metal of the second metal material that plastically flows is collected in an annular space formed by an outer circumference of the pin member, the distal end surface of the tool body and an inner circumference of the upper-side member of the clamp, and then a distal end of the pin member is dug into the first metal material to a predetermined depth position in the first metal material; and the working step further being characterized by movements of the pin member and the tool body being stopped when at least one of a change amount of an electric current that is applied to a drive source for rotating the pin member and a change amount of an electric current that is applied to a drive source for axially moving the pin member becomes larger than a predetermined threshold in the working step.