A tread rubber forming method (S1, S2, S3) for spirally winding and superposing tape-shaped unvulcanized rubber strips (GS1, GS2, GS3, and GS4) on an outer side in a radial direction of a cylindrical body to form an annular tread rubber, the method (S1, S2, S3) comprising: a first step (S11, S21, S31) of winding and superposing a first rubber strip while moving the first rubber strip in an axial direction of the cylindrical body, to form a base layer; a second step (S12, S22, S32) of winding and superposing a non-electrically-conductive second rubber strip in a region on one side in the axial direction on an outer circumferential surface of the base layer while moving the second rubber strip in the axial direction, to form a first intermediate layer; a third step (S13, S23, S33) of winding and superposing a non-electrically-conductive third rubber strip on an outer circumferential surface of the first intermediate layer while moving the third rubber strip in the axial direction, to form a first cap layer; a fourth step (S14, S24, S34) of winding, in a region on another side in the axial direction relative to the first intermediate layer and the first cap layer, an electrically-conductive fourth rubber strip from a position of contact with the base layer to an outer circumferential surface of the first cap layer, to form an electrically-conductive layer; and a fifth step (S15, S25, S35) of winding and superposing, on an outer side in the radial direction of the base layer, the third rubber strip in a region on the other side in the axial direction relative to the electrically-conductive layer while moving the third rubber strip in the axial direction, such that a portion of the electrically-conductive layer is exposed to an outer side in the radial direction of the tread rubber, to form a second cap layer. a first step (S11, S21, S31) of winding and superposing a first rubber strip while moving the first rubber strip in an axial direction of the cylindrical body, to form a base layer; a second step (S12, S22, S32) of winding and superposing a non-electrically-conductive second rubber strip in a region on one side in the axial direction on an outer circumferential surface of the base layer while moving the second rubber strip in the axial direction, to form a first intermediate layer; a third step (S13, S23, S33) of winding and superposing a non-electrically-conductive third rubber strip on an outer circumferential surface of the first intermediate layer while moving the third rubber strip in the axial direction, to form a first cap layer; a fourth step (S14, S24, S34) of winding, in a region on another side in the axial direction relative to the first intermediate layer and the first cap layer, an electrically-conductive fourth rubber strip from a position of contact with the base layer to an outer circumferential surface of the first cap layer, to form an electrically-conductive layer; and a fifth step (S15, S25, S35) of winding and superposing, on an outer side in the radial direction of the base layer, the third rubber strip in a region on the other side in the axial direction relative to the electrically-conductive layer while moving the third rubber strip in the axial direction, such that a portion of the electrically-conductive layer is exposed to an outer side in the radial direction of the tread rubber, to form a second cap layer.