A vehicular lamp unit comprising a light guide body (30, 30A), and a light source (21a, 21b, 21A) configured to emit light guided in the light guide body (30, 30A), wherein the light guide body (30, 30A) includes a first rod-shaped light guide portion (33a, 33Aa) configured to guide light from the light source (21a, 21A) from a proximal end (BE 33a) thereof to a distal end (FE 33a) thereof, a second rod-shaped light guide portion (33b, 33Ab) configured to guide light from the light source (21b, 21Ab) from a proximal end (BE 33b) thereof to a distal end (FE 33b) thereof, and a plate-shaped light guide portion (34, 34A) disposed between the first rod-shaped light guide portion (33a, 33Aa) and the second rod-shaped light guide portion (33b, 33Ab), wherein the first rod-shaped light guide portion (33a, 33Aa) and the second rod-shaped light guide portion (33b, 33Ab) are juxtaposed with each other with a space therebetween, wherein the plate-shaped light guide portion (34, 34A) includes a front surface (FS 34, FS 34A) disposed on a front side of the vehicular lamp unit and a rear surface (RS 34, RS 34A) disposed on a back side opposite to the front side, and also includes a first end rim portion (EG1, EG1A) configured to be connected to an outer peripheral surface of the first rod-shaped light guide portion (33a, 33Aa), and a second end rim portion (EG2, EG2A) configured to be connected to an outer peripheral surface of the second rod-shaped light guide portion (33b, 33Ab), and wherein the rear surface (RS 34, RS 34A) of the plate-shaped light guide portion (34, 34A) includes a plurality of structures configured to diffuse the light, which is emitted from the light source (21a, 21b, 21A) and enters the plate-shaped light guide portion (34, 34A) from the respective first and second rod-shaped light guide portions (33a, 33Aa, 33b, 33Ab), to cause the light to exit from the front surface (FS 34, FS 34A) of the plate-shaped light guide portion (34, 34A), the plate-shaped light guide portion (34, 34A) has a thickness between the first and second end rim portions (EG1, EG1A, EG2, EG2A) thinner than thicknesses (T1, T2) of the first and second rod-shaped light guide portions (33a, 33Aa, 33b, 33Ab), characterised in that the first end rim portion (EG1, EG1A) and the second end rim portion (EG2, EG2A) of the plate-shaped light guide portion (34, 34A) each include a respective thin-walled portion (NP1, NP1A, NP2, NP2A) including a first neck portion (NP1, NP1A) and a second neck portion (NP2, NP2A), respectively, which extend along the first rod-shaped light guide portion (33a, 33Aa) and along the second rod-shaped light guide portion (33b, 33Ab), respectively, and the first neck portion (NP1, NP1A) and the second neck portion (NP2, NP2A) each have a thickness (T4, T5) thinner than the thickness of the plate-shaped light guide portion (34, 34A). wherein the light guide body (30, 30A) includes a first rod-shaped light guide portion (33a, 33Aa) configured to guide light from the light source (21a, 21A) from a proximal end (BE 33a) thereof to a distal end (FE 33a) thereof, a second rod-shaped light guide portion (33b, 33Ab) configured to guide light from the light source (21b, 21Ab) from a proximal end (BE 33b) thereof to a distal end (FE 33b) thereof, and a plate-shaped light guide portion (34, 34A) disposed between the first rod-shaped light guide portion (33a, 33Aa) and the second rod-shaped light guide portion (33b, 33Ab), wherein the first rod-shaped light guide portion (33a, 33Aa) and the second rod-shaped light guide portion (33b, 33Ab) are juxtaposed with each other with a space therebetween, wherein the plate-shaped light guide portion (34, 34A) includes a front surface (FS 34, FS 34A) disposed on a front side of the vehicular lamp unit and a rear surface (RS 34, RS 34A) disposed on a back side opposite to the front side, and also includes a first end rim portion (EG1, EG1A) configured to be connected to an outer peripheral surface of the first rod-shaped light guide portion (33a, 33Aa), and a second end rim portion (EG2, EG2A) configured to be connected to an outer peripheral surface of the second rod-shaped light guide portion (33b, 33Ab), and wherein the rear surface (RS 34, RS 34A) of the plate-shaped light guide portion (34, 34A) includes a plurality of structures configured to diffuse the light, which is emitted from the light source (21a, 21b, 21A) and enters the plate-shaped light guide portion (34, 34A) from the respective first and second rod-shaped light guide portions (33a, 33Aa, 33b, 33Ab), to cause the light to exit from the front surface (FS 34, FS 34A) of the plate-shaped light guide portion (34, 34A), the plate-shaped light guide portion (34, 34A) has a thickness between the first and second end rim portions (EG1, EG1A, EG2, EG2A) thinner than thicknesses (T1, T2) of the first and second rod-shaped light guide portions (33a, 33Aa, 33b, 33Ab), characterised in that the first end rim portion (EG1, EG1A) and the second end rim portion (EG2, EG2A) of the plate-shaped light guide portion (34, 34A) each include a respective thin-walled portion (NP1, NP1A, NP2, NP2A) including a first neck portion (NP1, NP1A) and a second neck portion (NP2, NP2A), respectively, which extend along the first rod-shaped light guide portion (33a, 33Aa) and along the second rod-shaped light guide portion (33b, 33Ab), respectively, and the first neck portion (NP1, NP1A) and the second neck portion (NP2, NP2A) each have a thickness (T4, T5) thinner than the thickness of the plate-shaped light guide portion (34, 34A).