Iran reach Asian Cup last eight
Ansari Fard flicked home the winner after 63 minutes of a game that was riddled with mistakes from two sides who largely cancelled each other out in a tedious midfield tussle.
Iran, who have now won both their matches and are seeking a first Asian Cup title since 1976, were the better team throughout despite being badly affected by a flu bug that laid low six members of the squad this week.
But despite their superiority, their midfield fulcrum of Javad Nekonam and Pejman Nouri rarely found the right pass.
The one occasion they did led to the only goal when Nouri crossed perfectly and the unmarked Ansari Fard scored.
Iran top the group with six points, followed by the United Arab Emirates and North Korea, who both have one point.
Iran coach Afshin Ghotbi was forced to make four changes from the side that beat Iraq, but praised his men afterwards.
"I want to congratulate my team, we were hit by a flu bug that left six players either sick or semi-sick and I hope they will recover and we will be fully fit again soon.
"North Korea are a very difficult team to play against, they defend very well and counter-attack dangerously, and this group was the Group of Death. It was the most difficult group of the tournament, and to achieve this result and win our first two games makes me very proud of my players.
"We are also capable of playing much better football and hopefully we will show that in the remaining games."
North Korean coach Jo Tong-sop, whose side now have to beat Iraq in their last game to stand any chance of going through, said his side were weak at set-pieces and throw-ins.
Asked if he would play a more attacking game against Iraq next Wednesday, he replied: "We will play our counter-attacking game to look for goals."
Despite the problems in the build-up Ghotbi thought his team had taken the lead after 28 minutes when Ansari Fard held off a challenge from Korean defender Ri Jun-il, and fired in a well-taken shot, but the linesman flagged for handball.
Referee Nawaf Shukralla disallowed the goal then booked Ansari Fard for protesting although TV replays failed to show any infringement.
North Korea, who drew 0-0 with United Arab Emirates in their opening match, were, as expected, largely content to defend in numbers but they had two good scoring chances and did come out of their defensive shell after falling behind.
Their first chance came after 17 minutes when striker Jong Tae-se smacked a powerful 25-metre free-kick straight at goalkeeper Mahdi Rahmati who could only parry the shot although the follow-up effort was smacked high over the bar.
The second came in stoppage-time at the end of the second half when Hong Yong-jo's fierce drive hit the top of the bar.