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 (GS1) while moving the first rubber strip (GS1) 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 (GS2) 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 (GS2) 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 (GS3) on an outer circumferential surface of the first intermediate layer while moving the third rubber strip (GS3) 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 (GS4) 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 (GS3) in a region on the other side in the axial direction relative to the electrically-conductive layer while moving the third rubber strip (GS3) 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 (GS1) while moving the first rubber strip (GS1) 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 (GS2) 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 (GS2) 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 (GS3) on an outer circumferential surface of the first intermediate layer while moving the third rubber strip (GS3) 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 (GS4) 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 (GS3) in a region on the other side in the axial direction relative to the electrically-conductive layer while moving the third rubber strip (GS3) 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 . <EOS>