PURPOSE: To secure good injection pressure even in a low load, low revolution zone as well as to aim at improvements in engine performance, by constituting an actual compression ratio in a plunger chamber of a fuel pump so as to make it alterable freely according to engine load and speed during driving.

CONSTITUTION: A pressure regulating piston 12 is installed in a plunger chamber 13 of a pump block 10, and this piston is driven by a cam 23 via a lever 22. And, this cam 23 is constituted so as to be displaced and driven by a mechanism varying according to engine load and speed via a link mechanism or the like. With this constitution, in proportion as an engine comes to a low load, low speed zone, the piston 21 is thrust in, and a compression ratio of a piston chamber 14, namely, the plunger chamber 13 is kept up so as not to drop lower than in case of a high load, high speed zone. Therefore, even in the low load, low speed zone, a high injection rate is securable so that a combustion state is improved, engine performance is also improved and, what is more, a rate of fuel consumption is reduced, thus a problem on black smoke or the like is solvable.