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Japanese "Knee" Mortar - Legion of Valor

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Japanese infantry 50 mm type 89 light mortar (grenade launcher)

Japanese Knee mortar at the Legion of Valor Museum, Fresno, California

Japanese Knee mortar on display at the Legion of Valor Museum

This simple mortar had a short rifled barrel with and adjustable threaded rod inside. Used by the the Japanese infantry, it always fired at a 45 degree angle. The range of the projectile was adjusted by moving the rod inside the barrel, thus altering the space inside for combustion.

The bottom has a small curved plate to dig into the ground as seen in photo above (left end), and the launch tube is on the end to the right. A mortar shell can be seen to the left of the mortar.

Near the center of the top photo, is a WW2 Japanese mortar compass, used by the Japanese soldiers to direct range and the direction of fire and in the foreground is the mortar’s carrying case which would have been strapped to the leg when carrying the mortar.

Knee mortar range adjustor and trigger

Range adjustor and trigger.

50 mm Mortar Statistics

Weight of grenade 1.43 lb
Max range 700 yards
Weight of mortar 10.05 lb

Photo to the right shows a closeup of the mortar's range adjustor and the trigger (lower, middle of photo). There were several types of ammunition which could be used by the knee mortar including a 50mm shell and the type 91 grenade. Shell types were designated with painted lines and the carrying case for the rounds held four shells.

About the Knee Mortar

The Japanese called the type 89 50mm a “Leg Mortar” because it was carried strapped to the leg. Allied reports translated this as “knee mortar” believing it was fired holding the curved plate against the thigh. How many allied servicemen broke their thighs firing captured examples is unknown.


Photos taken with permission, at the Legion of Valor Museum in Fresno, California by Time Money and Blood staff. Text based on information from museum signs.

A description of this knee mortar in action can be found in Sides’, Ghost Soldiers.¹ Here Sides describes a Japanese soldier firing mortar rounds at the US Army Rangers who were on a mission to rescue the survivors of the Bataan Death March. Doctor Fisher was mortally wounded during the mortar attack.

¹Hampton Sides, Ghost Soldiers. New York: Doubleday, 2000, p. 283.