Lack of qualifications, low pay or tough working conditions are often cited as reasons.
IT is one sector in which jobs are often not filled because of a lack of skilled applicants.
Other struggling sectors include industry, building, sales, services to businesses and in the home, and farming work such as grape-picking and harvesting.
Up to 300,000 jobs went unfilled during 2017, the last year for which full figures are available from Pôle Emploi.
The main problem was lack of candidates with the right, or sufficiently up-to-date and specialised, training.
Jobs requiring technical knowledge, such as manufacturing industrial equipment, are among those where firms have great recruitment difficulties.
Under-qualification is also a problem: half of French jobseekers have not completed any post-baccalauréat training.
The problem is compounded by the fact that some seemingly straightforward jobs are becoming more technical – a refuse lorry driver, for example, now needs IT skills to operate the onboard computer used to organise the rounds.
Partly to blame is a lack of good continuing training, according to the OECD group of advanced economies.
Chief economist of Natixis Bank Patrick Artus says lack of skills is also holding back the use of advanced robotics technology.
He said, in Le Monde, it mostly explains why France has more unemployment than Germany.
Another issue in some sectors, such as hospitality, is that salaries and working conditions can be off-putting.
Figures from Insee and Pôle Emploi show specific jobs which are hardest to fill include dentists, technical draftspeople, panel beaters, pipe fitters, aircraft crew, roofers, carpenters, home help, machine regulators (who check settings), boilermakers and metalworkers.
Sectors where lack of skills is most cited as a problem include building, such as bricklayers (as well as secondary trades including electricians, plumbers and insulators), the motor industry and road haulage.